IPv6 IP Address Calculator
Size prefixes, compare subnet allocations, and review address ranges for home labs, VLANs, WAN links, and ULA planning.
| Prefix | Host Bits | Total Addrs | Typical Use |
|---|---|---|---|
| /48 | 80 | 2^80 | Site block |
| /56 | 72 | 2^72 | Branch PD |
| /64 | 64 | 2^64 | SLAAC LAN |
| /127 | 1 | 2 addrs | Router pair |
| /128 | 0 | 1 addr | Loopback |
| Range | Type | Use | Notes |
|---|---|---|---|
| 2000::/3 | Global | Internet | Routable |
| fc00::/7 | ULA | Private | Local scope |
| fe80::/10 | Link-local | Segment | No routing |
| ff00::/8 | Multicast | Group | Many listeners |
| 2001:db8::/32 | Docs | Examples | Reserved block |
| Parent | Child | Subnets | Planning Note |
|---|---|---|---|
| /48 | /64 | 65,536 | Great for sites |
| /56 | /64 | 256 | Small branch |
| /64 | /127 | 2^63 | P2P link pool |
| /64 | /128 | 2^64 | Single routes |
| /56 | /60 | 16 | Chunked LANs |
| Checkpoint | Value | What It Means | Hint |
|---|---|---|---|
| SLAAC | /64 | Host LAN default | Keep it simple |
| P2P | /127 | Two endpoints | No waste |
| Loopback | /128 | One address | Route only |
| ULA | fc00::/7 | Private scope | Local first |
| Docs | 2001:db8::/32 | Examples only | Safe samples |
Tip: Use /64 whenever hosts need SLAAC or normal LAN growth.
Tip: Use /127 for router pairs, and /128 for a single loopback route.
This calculator keeps the parsed address, the parent allocation, and the target prefix separate so you can see what actually fits before you publish a plan.
Because IPv6 addressing use different prefix sizes for different purposes for a network, when you recieve a block of addresses in the /56 range from your ISP, you must determine how many subnets of the /64 range you can create from your /56 block of addresses. The /64 prefix size is use for local area networks (LANs) because it permits a feature called stateless address autoconfiguration, or SLAAC. By not using the /64 prefix range for your LAN, your devices wont function correct.
Different types of networks requires different size prefixes for the best efficiency and functionality of the network. For instance, a point-to-point link between two routers will use a /127 prefix as it only requires two address for those two routers. Using a /64 prefix for a point-to-point network is inefficient in that it provides a very large amount of addresses for the link that do not require those many addresses.
How to plan IPv6 subnets
Additionally, very large sites will recieve a /48 prefix from there ISP so that they can create many subnets to manage there many different networks. Finally, unique local address (ULA), which are in the fc00::/7 range, will be used for private networks so that a person can maintain their own private network without creating conflict with the global addresses. When planning a network that will use IPv6 addressing, there is several considerations that must be made.
For instance, one consideration is the parent allocation and the target child prefix. The parent allocation is the total amount of IPv6 addressing space that you will receive (such as a /56). The target child prefix is the size of each subnet that you will create (such as a /64).
Additionally, you must also consider the number of devices that will be on each network and provide some growth reserve for the network so that you dont run out of subnet for your devices. A common mistake when planning IPv6 subnet is to treat IPv6 like IPv4 addressing. For example, when creating an IPv4 network, individuals may subnet their networks very aggressive to save on the number of addresses that are allocate.
In IPv6, however, you should of not subnet aggressively as the devices on the network will expect to find the /64 prefix on their local network. If you use a smaller prefix than a /64 on a local area network, then many network feature will no longer work correct. Additionally, do not use a /64 prefix for point-to-point links as this will waste a more massive amount of addresses for your network.
In order to understand IPv6 addressing, it is helpful to use reference tables to understand the different types of IPv6 addresses. For example, a reference table can show that a site block will use the /48 prefix, /128 prefixes will be used for loopback addresses, the fe80::/10 range of addresses will be used for link-local addresses (which will never leave the network wire), and it is important to plan for growth so that the network will not have to be renumbered if it expands; therefore, a growth reserve should be provide in the form of a 25% reserve, for example.
