Deploy360 24 September 2013

IPv6 Address Planning: Guidelines for IPv6 address allocation

1       Introduction

IP address planning is certainly among the critical functions required within the overall process of planning and executing an IPv6 deployment project. After all, one needs IPv6 address space in order to implement IPv6 and current IPv4 space must be managed in conjunction with the addition of IPv6 space. But the overall IPv6 deployment process requires not only IPv6 address space but an assurance that network and computing infrastructure is capable of processing and supporting IPv6 address space.

Please visit our page on IPv6 Address Planning for additional resources similar to this document.

At a high level, the deployment process generally requires a computing and networking assessment, which entails the validation of IPv6 support for your network infrastructure, devices, and applications. Such validation starts with an accurate inventory of said infrastructure, devices and applications. If you already maintain up-to-date network documentation including this information, you have already completed the first step. Otherwise, various forms of discovery may be required to identify and catalog these elements of your network. While you’re performing device and application discovery, it’s a good idea to also discover and document your current IPv4 address space as well which will come in handy later.

With an itemized network infrastructure in hand, the next step in planning your deployment consists of identifying IPv6 capability. This step may rely on respective vendor statements of compatibility or support at least initially, but ultimately will require testing prior to production rollout. For each network and computing component, you’ll need to validate IPv6 capability or determine mitigation steps required to bring the current state of a component to IPv6 capability. Such mitigation may entail a software upgrade, replacement or supplementation, e.g., in the case of supplementing DHCPv4 servers with DHCPv6 servers.

Upon completion of your review and assessment of your computing inventory, you’ll end up with two lists: one consisting of your IPv6-ready components and the other of your IPv6 “to do” list itemizing those elements requiring upgrade, replacement or supplementation. IPv6 has been around for quite some time, at least as a set of specifications, so vendors have had time to implement IPv6 support and hopefully most if not all of your components are IPv6 capable today. If not, you may have to prioritize your “to do” list based on the magnitude of the list along with your available resources. This may require scoping of your deployment to a finite region of your network, which is probably a good idea in any case to deploy initially within a controlled scope.

You’ll then be able to create a project plan based on your scope and “to do” list, along with lab testing to validate IPv6 capability and to prepare operationally prior to production. Your plan also needs to include your strategy for allocating your IPv6 address space and for managing your IPv4-IPv6 network. Plan to obtain an IPv6 address block from your Internet Service Provider (ISP) or Regional Internet Registry (RIR). With an IPv6 address block in hand, how should you carve this up to support your IPv6 deployment? Certainly, you’ll need to allocate IPv6 space in a manner that provides IPv6 address availability to those infrastructure and computing devices requiring IPv6 communications. This is where your IPv4 address space documentation or discovery can help in identifying IP address capacity requirements. But before you carve up your IPv6 address space in a manner analogous to your IPv4 space, consider the many other implications of IP address allocation and network management, discussed next.

2       Addressing impacts on managing your network

Besides assessing the ability of your current IPv4 network devices and applications to support IPv6, you’ll also need to think about how you will manage a dual protocol IPv4-IPv6 network. Management includes not only monitoring, fault detection and resolution, but also provisioning, performance management, and security. For each management system in use within your network for these respective purposes, consider its IPv6 abilities. You may need to upgrade, replace or supplement your management and security systems as well. But your IP address plan also plays a critical role in the ongoing manageability of your network.

2.1     Routing performance

Besides analyzing and forwarding IP packets across your network, routers also communicate among themselves regarding reachability of destination networks and nodes. Use of a routing protocol like Open Shortest Path First (OSPF) or Border Gateway Protocol (BGP) enables routers within a given network to identify optimal routes for IP packets across your network and to reroute around congestion or failure points.

Thus routers need to communicate frequently to identify and reroute around network issues. In each such communication, routers generally communicate reachability metrics for each network to provide information to peer routers for making routing decisions. If your network is modestly sized with less than a few hundred subnets, routing performance may be of minor concern. However for larger networks, the volume of “routes” communicated in each routing protocol exchange literally reduces the router cycles available to process user IP packets. In addition, processing of IP packets may require more cycles if routing tables updated by these routing protocols are correspondingly large. For such networks, your IP address plan can help improve routing efficiency by supporting hierarchical allocation.

Many modest to large networks are architected with a tradition three layer model, with an access layer comprising routers directly serving end users, which feed into routers within a regional layer which handle primarily intra-regional routing with escalation to routers within a core layer, which interconnect regions.  A hierarchical address plan allocates large blocks for each core router. Each regional router is allocated space from its respective core address block. Each access router is in turn allocated space from its respective access router.

Example Router Topology

Figure 1: Example Router Topology

Consider the example network of Figure 1. In this example router Rc is a core router. It and its core peer routers each serve a number of subtending regional routers. In Figure 1, Rc supports regional routers Rr1 and Rr2. Each regional router supports one or more access routers as illustrated in the figure with regional router Rr1 serving access routers Ra11, Ra12 and Ra13. Let’s say my ISP allocated address block 2001:db8:1a::/48 for my network. Under a hierarchical allocation model, I would allocate a large block to each core router, say block 2001:db8:1a:1000::/52 for router Rc. This block then serves as the pool for subtending access routers. For example, I may allocate 2001:db8:1a:1000::/56 and 2001:db8:1a:1100::/56 to routers Rr1 and Rr2 respectively. In turn, each regional router block is further allocated for subtending access routers. I may allocate for example a /64 for each access router interface, each of which supports a subnet for end user devices. I can allocate router Ra21 the network 2001:db8:1a:1110::/60, from which I can assign subnet addresses for router interfaces, e.g., 2001:db8:1a:1110::/64, 2001:db8:1a:1111::/64, 2001:db8:1a:1112::/64, etc.

This hierarchical allocation model enables my core routers to communicate just one route each, in this case of size /52, in its routing updates. Each of my regional routers can likewise communicate a single route, a /56 sized network in its routing update. And finally each access router communicates reachability to its /60 network. As you can see, the “roll up” of address space hierarchically leads to optimal efficiency in routing tables and routing protocol communications.

2.2     Router policies based on IP addresses

Routers leverage routing tables stored in memory to make routing decisions. These routing tables are updated by frequent routing protocol communications with peer routers. Routers generally may also be configured to use additional information to make routing decisions, particularly based on information in the header of each IP packet it processes, including source and destination IP addresses. The router may drop packets from certain source IP addresses, may prioritize packets of with a given destination address or other header parameter values, or otherwise treat packets based on header parameter values.

For example, some organizations allocate separate address space to be assigned to voice over IP (VoIP) devices as contrasted to data devices. An IP packet incoming to a router with a source IP address assigned from the VoIP space should be treated in accordance with the VoIP policy. Similarly, a packet with source IP address from the data space should be treated as configured for the data policy.

In this case of configuring routing policy based on the source or destination IP address, your IP addressing plan will directly impact the simplicity or complexity of defining such policies. Building on our network example, let’s decide to allocate each access router’s /64 subnet in accordance with an “even-dd” policy where an “even” address is a VoIP address and an “odd” address is a data address. For example on “interface 1” for router Ra21, I will allocate 2001:db8:1a:1110::/64 for VoIP users on this interface and 2001:db8:1a:1111::/64 for data users on this interface. For “interface 2” on router Ra21, I can allocate 2001:db8:1a:1112::/64 for VoIP users on this interface and 2001:db8:1a:1113::/64 for data users on this interface, and so on. Simple enough. The issue now becomes one of ongoing management and inevitable policy changes required over time. If I need to change a router parameter for how VoIP packets are treated, I will need to create a policy entry for each “even” subnet in my network in the worst case! If you have hundreds of routers and/or interfaces, you can imagine the potential complexity.

2.3     Security functions

Much like the definition of IP address based routing treatment policies, security policies can be scoped based on IP addresses. Filtering of packets based on source or destination IP addresses and perhaps other header information such as port number is common practice. Hence your IP address plan will have an impact on the day-to-day manageability of configuring and updating security policies.

Configuring access control lists (ACLs) based on IP addresses can be simplified if the address plan is defined such that ACLs can be defined with just a few entries in the ACL configuration of the router, firewall, DNS server or device in question. Ongoing security operations may require the need to redirect traffic, isolate or quarantine a site known to have an active virus or bot, or to “zoom in” on a particular traffic flow for troubleshooting or analysis. In our example, each access router was allocated a /60 block. If each access router represents a site, this allocation strategy facilitates the quarantining of a given site from a security perspective by blocking traffic into the regional routers from the site’s /60 network.

3       Goals of an IPv6 address plan

As we’ve demonstrated, your IP address plan is vital not only to making sure network devices are uniquely addressed, but also to facilitating ongoing management of your network. We’ll walk through specific examples illustrating these concepts in more detail, but first let’s review the goals of an IP address plan.

1)   Provide address capacity

The fundamental goal of an IP address plan is to provide IP addresses to infrastructure and end user devices. Without an IP address, a device by definition cannot communicate on an IP network. Of course you may want to control which devices obtain IP addresses and also structure your IP address space to streamline routing performance.

2)   Enable end nodes to communicate (or not)

Not every device obtaining an IP address on your network should necessarily have capability to communicate with any other IP address on your network. Controlling access to sensitive systems and applications is a network management necessity and is typically based on IP addresses, e.g., controlling which source IP addresses can reach given destination IP addresses. And in the age of bring your own device (BYOD), controlling access from end user devices could represent an additional layer of access controls.

3)   Enable Internet communications (or not)

Most enterprises enable internal users to communicate to Internet hosts, though the set of destination hosts may also be constrained due to security or corporate policies implemented in Internet routers. BYOD devices or visitors may be granted access only to Internet destinations.

Inbound communications from the Internet is generally controlled through the deployment of a “demilitarized zone” (DMZ) where external facing web, email, DNS and other Internet servers are deployed “in front of” an enterprise network gateway which constrains access from Internet source addresses. Thus these policies are likewise dependent on the IP address plan.

4)   Enable communications via supported applications

IP networks support multi-media communications though different media impose different traffic flow and response time requirements. Voice communications generally require low latency though occasional intermittent packet drops are tolerable; meanwhile, data communications are more tolerant to higher latency but less so to packet drops in general, though this also depends on the particular data application in question.

Satisfying media-specific communications requirements generally involves configuration of routers to provide application specific treatment. And in many cases, the router is configured to recognize a given application for treatment based on the source and/or destination IP addresses in each IP packet it processes.

5)   Facilitate visual mapping of IPv6 address to location, application, node, etc.

If you’ve been managing your IPv4 network for a while, you can probably classify an IPv4 address by sight with respect to its respective location, application or even node type (router, switch, server, etc.). A well-structured IP address plan promotes this ability to visually identify node properties from its IP address, as this can help improve manageability and reduce troubleshooting time.

While it may seem simple to map the value of up to four decimal numbers within a dotted-decimal IPv4 address to a given property, mapping a much lengthier hexadecimal IPv6 address may seem intimidating. However, consider that every IP address in your network will share the same prefix, e.g., 2001:db8:1a::/48 per our earlier example. The address prefix from your ISP or RIR will be common on all IPv6 addresses unless you are deploying address spaces from one or multiple ISPs or RIRs and/or unique local address (ULA, akin to private) space. And the second half of each IPv6 address, bits 65-128, comprises the node’s interface identifier (IID). So the bits between your prefix length and bit 64 comprise those bits that define your network blocks or subnets.

In our example 16 bits, bits 49-64 within each 128-bit IPv6 address can be used within our address plan to define our blocks and subnets. These sixteen bits represent four hexadecimal digits, so as you define your plan, a goal should be to map each of these four hex digits to significance that can be visually recognized (over time as you get comfortable with IPv6) as relating to location, application, security, etc.

4       IPv6 address allocation hierarchy and techniques

Now that we’ve seen how your IP address plan impacts the manageability of your network and we’ve discussed the goals you should consider when developing your IPv6 address plan, let’s delve into the mechanics of performing IPv6 allocations. While you should apply a common allocation technique at each layer of your allocation plan, you may apply different techniques at different levels of your address plan hierarchy.

Recall in our example, we defined four layers, starting with a source IPv6 block of 2001:db8:1a::/48, then allocating /52 blocks for the core, /56s for regions, /60s for access/sites and finally /64s for subnets.  We could apply the sparse technique for the core layer, best fit for regions, random for sites, and monotonic for subnets for example.

4.1     Monotonic allocation

The monotonic allocation scheme merely allocates subnets, counting up from block address 0, 1, 2, etc. If we applied this method to our core layer, we would first allocate block address 0, 2001:db8:1a:0000::/52 (i.e., 2001:db8:1a::/52), then block address 1, 2001:db8:1a:1000::/52, then 2, 2001:db8:1a:2000::/52, etc. This method is the simplest method and merely counts from 0 to f, providing up to 16 blocks at this level. Figure 2 illustrates this allocation method representing our source block and allocations within a pie chart.

Monotonic IPv6 Allocation

Figure 2: Monotonic Allocation

If you require more than 16 allocations at a given level, you should allocate 2 hex digits (nibbles), providing up to 65,536 blocks. While there is a wide disparity from 16 to 65,536, allocating on nibble boundaries is highly recommended by the Internet community and RIRs to simplify the allocation process and reduce allocation errors. Nibble allocations also simplify delegation of respective reverse DNS zones, which are represented as dot-separated hex digits within the domain.

4.2     Sparse allocation

RFC 3531 describes the sparse allocation technique, which entails monotonic counting, but from the leftmost bits instead of the rightmost bits. Using a four-bit example, rightmost counting yields 0000, 0001, 0010, 0011,… or 0, 1, 2, 3,… as we saw in the prior section. Leftmost counting yields 0000, 1000, 0100, 1100,… or 0, 8, 4, c,… which essentially mirrors the rightmost count bitwise. The effect of this technique is to spread out allocations as much as possible as illustrated in Figure 3.

Sparse Allocation

Figure 3: Sparse Allocation

The motivation for such a scheme is to allocate space while leaving room for future growth. For example if core router 1 Rc1 in Figure 3 requires more address space in the future, in all likelihood, a contiguous block will be available (unallocated), so we could allocate the 2001:db8:1a:9000::/52 block. The benefit is that Rc1 need not advertise another block due to this, as we can merely change the prefix length of the block already being advertised. Since blocks 2001:db8:1a:8000::/52 and 2001:db8:1a:9000::/52 can be joined to form a single block, 2001:db8:1a:8000::/51, we need only adjust the prefix length of Rc1’s advertisement. This serves the purpose of adding needed IP address capacity without adding a route to router routing tables or routing protocol updates.

4.3     Best-fit allocation

You may be familiar with the best-fit allocation method in trying to squeeze every IPv4 block available in an optimal manner. Classless Inter-Domain Routing (CIDR) introduced the concept of variable sized non-octet bounded network addressing to more precisely meet address capacity requirements while conserving IPv4 address space. With the vast IPv6 address space, this may not be of a concern at least today. In general, you should keep things simple and allocate same-sized blocks at each layer of your hierarchy if possible. However, to illustrate this technique, Figure 4 shows an example of best-fit allocation, which entails allocation of the smallest available block to meet the demand requested. In the figure, we’ve allocated three /52’s for three of our routers, but Rc3 requires a larger allocation, a /51 (again, nibble boundary allocations are recommended!) and is allocated  2001:db8:1a:4000::/51.

Best Fit Allocation

Figure 4: Best Fit Allocation

From this state of Figure 4, if the next allocation called for a /52, the wedge between the allocations, 2001:db8:1a:3000::/52 could be allocated; if a /51 was needed, 2001:db8:1a:6000::/51, contiguous with our already-allocated /51, could be allocated.

4.4     Random allocation

A random allocation mechanism features randomizing the bits for the allocation layer in question. In our example, we are allocating 4 bits to our core layer. Define a random number between 0 and 15 (f) and if not already allocated, allocate this block address. Figure 5 illustrates an example of the random allocation in pie chart form. For my first router Rc0, my random number was 5, for Rc1, 8 and for Rc2, 15 in this example. The random allocation scheme may best apply to sites or subnets in order to avoid monotonic block counting, to make it more difficult for someone to footprint or discover your IP allocation plan from a security perspective.

Random Allocation

Figure 5: Random Allocation

4.5     Prefix delegation

Prefix delegation is not an allocation technique per se, but it provides a mechanism to automate allocations within the network using the DHCPv6 protocol. A router or DHCPv6 server configured with a pool of blocks can allocate entire blocks to downstream requesting routers. A service provider desiring to allocate /56 blocks to customers might use prefix delegation to automate this process upon provisioning for example.

5       Allocation use cases/examples

Having now defined our allocation techniques, let’s now turn to some examples. Section 2 introduced one example allocation scheme based primarily on mimicking routing topology. In this section, we will review and contrast two example allocation strategies to help convey the trade-offs in defining the order or allocation.

5.1     Case 1

Let’s start with our source IPv6 block, 2001:db8:1a::/48. We will allocate space successively for the following layers: application, location, business unit and site.

5.1.1     Allocations

I will perform my first layer allocation by application, VoIP, video, data, wireless, etc. This will enable me to define one routing policy across all of my routers for each application as appropriate. I have less than 16 allocations, so let’s allocate the first nibble, bits 49-52 as follows. I am also using the sparse allocation technique to allow room for growth.

–        Data: 2001:db8:1a:0000::/52

–        Voice: 2001:db8:1a:8000::/52

–        Video: 2001:db8:1a:4000::/52

–        Wireless: 2001:db8:1a:c000::/52

–        Management: 2001:db8:1a:2000::/52

My single video routing policy statement to be configured in all of my routers need only enumerate application to packets with source IPv6 address within the 2001:db8:1a:4000::/52 block.

Next I’d like to allocate my second layer by region, taking the next nibble or four bits. These I will also allocate sparsely. For each application block defined above, I now need to allocate by region. Consideration our Voice block, our regions might be allocated as follows.

–        Voice – Eastern region:    2001:db8:1a:8000::/56

–        Voice – Northern region: 2001:db8:1a:8800::/56

–        Voice – Western region:  2001:db8:1a:8400::/56

–        Voice – Southern region: 2001:db8:1a:8c00::/56

I would repeat the allocation of four regions within each application block, assuming each application is supported in each region. With 5 blocks at my top layer and 4 in the second layer, I will have made up to 20 allocations so far.

Now I want to define a third layer, this one by business unit, and I’ll use the next nibble, bits 57-60. Assuming each business unit requires representation in each region and for each application, I need to add each business unit to my 20 existing allocations. So if I have five business units, that’s 100 allocations! Let’s look at the Voice – Southern region branch and add our business unit allocations monotonically:

–        Voice – Southern region – Corporate: 2001:db8:1a:8c00::/60

–        Voice – Southern region – Finance: 2001:db8:1a:8c10::/60

–        Voice – Southern region – Marketing: 2001:db8:1a:8c20::/60

–        Voice – Southern region – Engineering: 2001:db8:1a:8c30::/60

–        Voice – Southern region – Customer Support: 2001:db8:1a:8c40::/60

I have four bits remaining which I can allocate to specific subnets for each business unit with each region for each application. For example if I have 3 subnets for my Marketing team for voice service in the southern region, I could allocate randomly:

–        Voice – Southern region – Marketing – Site 1: 2001:db8:1a:8c2a::/64

–        Voice – Southern region – Marketing – Site 2: 2001:db8:1a:8c29::/64

–        Voice – Southern region – Marketing – Site 3: 2001:db8:1a:8c2e::/64

5.1.2     Observations

As you may have observed as we built out our example, successively drilling down to maintain brevity, our subnet bits, particularly bits 49-64, do provide a visual mapping to a given subnet’s application, region, business unit and site. In our final allocation, you can see that the subnet “8c29” is for voice, southern, marketing and site 2. If we’ve maintained uniformity in our allocations across parallel branches, we can also readily state that subnet “403e” applies to video (first nibble, “4”), east (“0”), engineering (“3”) and site 3 (“e”) though with a random allocation my site number may not map analogously. But in general the visual mapping goal has been achieved, and over time, you will recognize each by sight!

I could generally conclude that we should be able to achieve the goal of providing communications by our supported IP-based applications, given this criterion comprised our first layer. And my routing policy implementation and management is quite simple given one entry (IPv6 block) per application. Assuming I’ve identified my regions and sites according to my current network reach and capacity requirements, it’s likely I can check off the goal of providing IP address capacity. In terms of route efficiency, assuming routers are conveyed at the region level, my core routers will require at least 20 routes in this example.

As for the goal of selectively enabling or disabling communications among nodes internally and to or from the Internet, my achievement of this goal is not so obvious. This is where you should consider those criteria that drive your filters and ACLs regarding constraining access by IP address. If you have a frequent need to throttle applications by policy or set security policies by application, then this allocation scheme achieve this with relative simplicity, with one block defined for each application across my network. Setting such policies for a given region will require 5 statements (a given region is defined in each of the 5 application blocks) in our example. In general setting second layer policies upon a single nibble first layer requires up to 16 entries. Setting such policies by business unit is more onerous, requiring setting of only 20 in our example but up to 65,536 (two nibble’s worth) of policies to define the business unit traffic per region and per application.

5.2     Case 2

Let’s start again with our source IPv6 block, 2001:db8:1a::/48. This time, we will allocate space successively for the following layers: business unit, application, location, and site.

5.2.1     Allocations

Following similar logic to that discussed above, let’s allocate as follows for our top layer, business unit:

–        Corporate: 2001:db8:1a:0000::/52

–        Finance: 2001:db8:1a:1000::/52

–        Marketing: 2001:db8:1a:2000::/52

–        Engineering: 2001:db8:1a:3000::/52

–        Customer Support: 2001:db8:1a:4000::/52

Our next layer is application, so considering application allocations within our Engineering block:

–        Engineering – Data: 2001:db8:1a:3000::/56

–        Engineering – Voice: 2001:db8:1a:3800::/56

–        Engineering – Video: 2001:db8:1a:3400::/56

–        Engineering – Wireless: 2001:db8:1a:3c00::/56

–        Engineering – Management: 2001:db8:1a:3200::/56

We allocate by region as our third layer, so drilling into Engineering – Data, we have:

–        Engineering – Data – Eastern region:    2001:db8:1a:3000::/60

–        Engineering – Data – Northern region: 2001:db8:1a:3080::/60

–        Engineering – Data – Western region:  2001:db8:1a:3040::/60

–        Engineering – Data – Southern region: 2001:db8:1a:30c0::/60

And finally by site, considering our Engineering – Data – Northern region block:

–        Engineering – Data – Northern region – Site 0: 2001:db8:1a:3080::/64

–        Engineering – Data – Northern region – Site 1: 2001:db8:1a:3081::/64

–        Engineering – Data – Northern region – Site 2: 2001:db8:1a:3082::/64

5.2.2     Observations

As in Case 1, our nibble based allocation has again yielded a visually discernible classification of a subnet by business unit, application, region and site, which should generally facilitate manageability. We may also surmise that our goals of providing for capacity and applications use have likewise been satisfied.  Our route efficiency, again assuming a route is required at the core for each regional level, has suffered as we now have 100 unique region entries, one per application per business unit.

Regarding our routing and security policies, this case certainly simplifies setting of policies by business unit, with one block for each. This comes at the expense of increasing the effort to institute application based policies and further complicates regional based policies. In general, the number of policy entries at each level for a given criterion is calculated as the product of the number of allocations of each layer above in the allocation hierarchy.

6       General block allocation guidelines

As you begin preparing your IPv6 address plan, keep in mind there is no “cookie-cutter” solution and each address planner must consider several factors as illustrated above. Think about your router topology and relative importance of constraining router table sizes. Consider the applications supported by your network, particularly those utilizing IP-based routing treatment. Identify and prioritize your frequent management and security processes and requirements. Review the goals we discussed earlier and define those that are most important to you.

In terms of allocation layers and address block structure, you should consider your current IPv4 structure as a data point, not necessarily as a model. You may wish to allocate an IPv6 subnet for every IPv4 subnet you’ve allocated, but you may be able to consolidate subnet addresses given the vast size of a given /64 IPv6 subnet with capacity for 1.8 X 1019 addresses.

Your current IPv4 address space also provides useful information regarding IP address capacity requirements at the host level, helping define how many addresses are required in given locations and by application or media type. In general, it’s a good idea to baseline your IPv4 address plan as input to the IPv6 plan, again as a data point, not necessarily as a template.

The following is a summary of key guidelines when defining your IPv6 address plan.

  • Baseline your IPv4 address space in terms of documenting your IP blocks, subnets, host assignments and DHCP pools. If you’re using an IP address management (IPAM) system, this information should be readily available. Otherwise you may need to run network discoveries to confirm and validate IP address allocations.
  • Define all of your IPv6 address space(s) to be allocated. So far we’ve talked about public space from your ISP or RIR and briefly mentioned unique local address (ULA) space. Also consider if you use or plan to use any IPv4-IPv6 co-existence technologies besides dual stack including tunneling or translation techniques, most of which have implications on IP address assignment and formats. Your address plan should enumerate each use of IPv6 address space.
  • Identify your network routing and switch infrastructure and document your network architecture, e.g., routing layers per the discussion in section 2. Determine the level of importance of routing table sizes in your environment. The more critical for you to keep routes contained, the higher a router level allocation layer should be in your plan’s layers.
  • Enumerate IP address based policies in use or planned for your network. This includes routing policies, packet treatment policies, filtering, ACLs, and other IP address based configuration parameters within your infrastructure. Determine the rate of change or urgency to enact policies at a given level. If you plan to setup application based policies once and plan no or minimal changes, then your application allocation should you choose to include such a layer, may be deeper in your allocation plan. For those frequently changing policies or for potentially urgent requirements (e.g., to quarantine a site), consider placing allocations encapsulating the scope of these policies higher in your allocation plan.
  • Define the homogeneity of your network. In our use cases, we assumed all applications were to be available at all locations for all business units. If this is not the case, then those more universal criteria should be higher than those more limited in scope in your allocation layers.
  • Consider administrator delegation and responsibilities, especially if defined by IP addresses, and how frequently or urgently these are changed.
  • Identify DNS configurations (zone files) recognizing that as you deploy IPv6, hosts will require IPv6 DNS resource records (AAAA and PTR among others). That is for a dual stack implementation a given hostname should have both A and AAAA records in the corresponding DNS zone. If you delegate DNS responsibilities within your organization, zone administrators may require training to properly configure DNS resource records and IPv6 reverse ( zones.
  • Evaluate alternative allocation techniques, monotonic, sparse, best-fit and random, then define your approach for each layer.
  • Plan to allocate on nibble boundaries to ease the allocation process, to provide visual address mapping and to simplify DNS configurations especially for delegated zones corresponding to IPv6 allocations.

7       ULA considerations

Unique Local Address (ULA) space in IPv6 is analogous to private address space in IPv4. Most IP planners have become accustomed to the common IPv4 addressing strategy of utilizing private RFC 1918 space internally to their networks, then through the use of proxies and/or network address translators (NATs), translate private address space to a smaller pool of public IP addresses for communication over the Internet. As such, NAT was an effective component in extending the lifetime of IPv4 as each enterprise demanded a smaller pool of public IPv4 addresses.

IPv6 does not suffer the scarcity issues of IPv4 and was designed to restore the endpoint-to-endpoint communications model originally intended for the Internet Protocol. Hence use of NAT for IPv6 is highly discouraged. While NAT functions may provide a sense of security by filtering and hiding internal addresses, IPv6 address footprinting is much more difficult given the sheer size of IPv6 space. Privacy extensions to stateless address autoconfiguration (SLAAC) also enable individual hosts to periodically change their addresses to further complicate the reconnaissance process for would-be snoopers. NAT devices also potentially interfere with end-to-end application performance and operation.

Public IPv6 addresses, otherwise known as global unicast addresses (GUAs), should be assigned to your devices. Filtering is recommended from a security perspective but not NAT. Hence a model using ULA internally NAT’d to public IPv6 is discouraged. Nevertheless, ULA space has utility for assignment to devices that don’t require Internet access or for use on private, partner and VPN connections.

8       Renumbering impacts

Another goal of an IP address plan not stated above but certainly in the back of every IP planner’s mind is to “get it right the first time,” so you don’t have to renumber your network. Renumbering of networks is generally a difficult and time-consuming project and should be avoided if at all possible. This is one reason I encourage you to take the time to develop your allocation plan, evaluate alternative strategies, and to understand the trade-offs made in your final strategy. Regardless, even with the best laid plans, unforeseen events occur and impact the network in such a way that a partial or complete renumbering is required.

An initiative as seemingly innocuous as changing ISPs may necessitate a complete renumbering of your network. But if you’ve done the analysis in diligently preparing your IPv6 address plan, such a change in IPv6 prefix may be a simple prefix substitution (assuming the same prefix length) within the address plan, though actual device re-addressing must also ensue. The IETF recognizes renumbering to be a difficult problem to solve as indicated in the title of RFC 5887, Renumbering Still Needs Work. This is due to the need to update devices with hard-coded IP addresses for communications, for use within APIs and configuration files and those used by network management and security systems as well.

Note that these steps are required for your initial IPv6 “numbering” project, so I advise you to document your steps diligently so you can refer to them later should a renumbering become necessary. Though of incomplete consolation, IPv6 does support new helpful features in renumbering, namely router advertisements, SLAAC, and address lifetimes. Suffice it to say that you should think through your IPv6 address plan to derive to best possible address plan for your network, and document your process for possible future reference.

9       Summary and conclusions

This paper has provided key considerations and techniques for defining your IPv6 address plan, a critical ingredient of your IPv6 deployment plan. Take the time to carefully consider your IPv6 address structure and strategy to develop an address plan to facilitate your IPv6 deployment and management of your network in steady state.

About the author

Timothy Rooney is the Product Management Director for BT Diamond IP which offers IP address management appliances, software and managed services to enable customers to effectively manage complex IP networks. Prior to joining BT, Tim held a comparable position at Lucent Technologies, and has worked for Comcast Cellular and AT&T Bell Laboratories. Tim has co-authored an IPv6 deployment and management book and authored two books on IP address management with Wiley/IEEE Press. Tim holds a B.S. in Commerce and Engineering Sciences from Drexel University and an M.S.E.E. from Rutgers University.

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