IPv6 Addressing Tips
Ross Chandler, Principal Network Architect of IP network evolution at Eircom/Meteor, shares a few tips on working with IPv6 from his own experience. The bottom line? You can do this!
Guest Blog Post by Ross Chandler
The most significant changes with IPv6 are: vastly more addresses and the way the extra bits are used. Here are a few practical tips for when you’re adding IPv6 to your network and connected devices.
Don’t stress about the length of IPv6 addresses
The long ones only occur when they’re generated automatically. Don’t attempt to read out one of these long addresses for another human being. You can assign shorter IPv6 addresses by static configuration or by DHCPv6.
Use the 4-bit nibbles when making an addressing plan
The 4-bit (hexadecimal) character positions makes subnetting easy.
e.g. Your assignment might be 2001:db8:1234::/48
This can be subnettied into 16 /52s (prefix length increased by 4)
Each of the /52s can be further subnettted into 16 /56s
And so on down to the /64s.
Combining contiguous nibbles allows a prefix to be subnetted into a larger number [16^(number of nibbles)] of smaller subnets with prefix length increased by 4 * (number of nibbles).
2001:db8:2014:1000::/48 can be subnetted into 16 /52 prefixes. 16 = 16^1 and 52 = 48 + 4 * 1.
2001:db8:2015:1200::/48 can be subnetted into 256 /56 prefixes. 256 = 16^2 and 56 = 48 + 4 * 2.
2001:db8:2016:1230::/48 can be subnetted into 4,096 /60 prefixes. 4,096 = 16^3 and 60 = 48 + 4 * 3.
IPv4 subnetting is not as simple as that.
Odd or even address conventions
If you use a /30 IPv4 subnet on a link then a /126 IPv6 prefix length will allow both the IPv4 and IPv6 address at either end to be odd or even. Similarly for /31 IPv4 or /127 IPv6 links.
You can be liberal with your use of IPv6 /64 prefixes
Don’t be afraid to be liberal when assigning /64s. It’s often helpful to think of 64 bit prefixes as the smallest unit of address assignment of v6. For example, assign a full /64 for each point-to-point link even if you intend using a /126 or /127 mask. This is all right because whether there are 1 or a 1,000 devices on the LAN, compared to the 2^64 possible addresses both are almost equally sparse. Stateless address autoconfiguration (SLAAC) mandates the use of a /64 prefixes on LANs. This fact and the :: compactor allows manually assigned IPv6 addresses to be written in short form with almost half the number of characters as a typical SLAAC assigned address.
Assigning more specific IPv6 subnets
You can make assignments with larger prefix lengths. For example, you may have IPv4 DNS server addresses 203.0.113.1 and 203.0.113.2 and so decide to use the first two addresses from your IPv6 allocation for your IPv6 DNS server addresses 2001:db8::1 and 2001:db8::2. The service number (e.g 53 for DNS) could be the host part of the address.
Principal Network Architect - IP network evolution
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