IPv6 /48 advertisements

Eric C. Miller eric at ericheather.com
Thu Dec 19 05:51:38 UTC 2013

Owen, thanks for this explanation. +1!

Eric Miller, CCNP
Network Engineering Consultant
(407) 257-5115

-----Original Message-----
From: Owen DeLong [mailto:owen at delong.com] 
Sent: Wednesday, December 18, 2013 2:07 PM
To: Cliff Bowles
Cc: nanog at nanog.org
Subject: Re: IPv6 /48 advertisements

On Dec 18, 2013, at 08:11 , Cliff Bowles <cliff.bowles at apollogrp.edu> wrote:

> I accidentally sent this to nanog-request yesterday. I could use some feedback from anyone that can help, please.
> Question: will carriers accept IPv6 advertisements smaller than /48?

Generally, no. Since a /48 should represent nothing larger than a single site, it's not very reasonable to want to route something longer in general.

> Our org was approved a /36 based on number of locations. The bulk of those IPs will be in the data centers. As we were chopping up the address space, it was determined that the remote campus locations would be fine with a /60 per site. (16 networks of /64). There are usually less than 50 people at the majority of these locations and only about 10 different functional VLANs (Voice, Data, Local Services, Wireless, Guest Wireless, etc...).

That's still poor planning, IMHO. You can easily get more than enough /48s to give one to each location. There's absolutely no advantage in the IPv6 world to being stingy with address space and no benefit to not putting at least a /48 at every location.

You've got 10 VLANs, so you're wasting at most 65,526 networks. Compare that to the fact that using a /64 for a VLAN with less than 2,000,000 hosts on it will wast at least 18,446,744,073,707,551,616 addresses and you begin to realize that sparse addressing in IPv6 and large amounts of excess address capacity are intentional.

> Now, there has been talk about putting an internet link in every campus rather than back hauling it all to the data centers via MPLS. However, if we do this, then would we need a /48 per campus? That is massively wasteful, at 65,536 networks per location.  Is the /48 requirement set in stone? Will any carriers consider longer prefixes?

Massively wasteful is a fact of life in IPv6. Consider it this way... There are two ways to waste address space. One way is, as you describe above, deploying it to locations that are unlikely to fully utilize it.

Another way is to leave it sitting in a free pool until long after the protocol is no longer useful.

With IPv6, we're not so much choosing between wasting address space or not. We're choosing how much address space gets wasted using method 1 vs. how much gets wasted using method 2. Ideally, we arrive at the protocol end of life with some space remaining in both categories of waste.

> I know some people are always saying that the old mentality of conserving space needs to go away, but I was bitten by that IPv4 issue back in the day and have done a few VLSM network overhauls. I'd rather not massively allocate unless it's a requirement.

It's a requirement and not massively allocating will bite you harder in IPv6 than space did in IPv4.

IPv4 was designed for a different kind of network. It was designed to support some labs and some institutional environments. It was never intended to be the global public internet. IPv6 has been designed with the idea of addressing absolutely everything from the ground up. The design allows for plenty of /48s to number every building that could possibly fit on every planet in the solar system and several other solar systems.

Frankly, a /48 per campus is underallocating for any campus that has more than one building.


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