IPv6 design, deployment, standards, and best practices.
lmccormick1922
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subnet size

Sat Feb 14, 2015 6:42 pm

With there being no broadcast in IPv6 and only Unicast, Anycast, and Multicast. Is there any real need to limit subnet size to the golden rule of 500 anymore?

Also, how does multicast without IGMP get treated? Still as broadcast style flood?

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maapinho
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Re: subnet size

Tue Feb 17, 2015 4:03 pm

Hello,
While there is no more broadcast in IPv6 there is still the "all hosts" multicast address which gets processed just like an ipv4 broadcast.

For a good post on the reasons to limit the number of hosts on a VLAN/subnet have a look at:
http://etherealmind.com/many-hosts-vlan-ip-subnet/

Can't comment on the 2nd part of your questions.

Fred
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Re: subnet size

Sat Apr 25, 2015 11:10 pm

RFC 3177 addresses the size of subnets.

To summarize, it's actually quite simple. All of your subnets should be /64's, regardless of the number of hosts. That point to point link that's a /31 in IPv4? It should be a /64. That backup network that talks to every host and has 10,000 endpoints, way beyond the recommendations? Still a /64.

Then, each site should be assigned a /48. This gives you 64-48=16 bits to use for subnetting. That's 65,536 subnets.

I'm still a bit confused on what it means if I have multiple sites with one edge. If I'm reading ARIN's guidelines correctly, you should request a larger supernet based on the number of sites. 1-11 sites gets a /44, 12-191 gets a /40, and so forth.

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srg
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Re: subnet size

Sun Apr 26, 2015 2:43 am

Fred wrote:That point to point link that's a /31 in IPv4? It should be a /64.
Please, no. There's some nice NDP table exhaust vectors there.
som om sinnet hade svartnat för evigt.

Fred
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Re: subnet size

Sun Apr 26, 2015 10:34 am

Looks like you're right.

RFC 3627 made the case that /127's were a bad idea, but it's been superceded by RFC 6164, which says /127's are good again.

I stand corrected. So, new recommendation: /64's everywhere, except point to point links, which should be /27's.

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