Max number of hosts on IPv6 VLAN

[andrecsthompson]

Hi everyone

Here's my question :

A single Ethernet segment supports about 500 hosts max because after that there will be broadcast storms.

Why then IPv6 is designed to support 2^64 hosts in one subnet or am I missing something?

Thanks.

- Andre

[scottpickett]

I think it's more of an IPv6 autoconfiguration concern vs IPv6 protocol design, but it does mean that you will be wasting 2^64 less 500+/- host bits per network if you use EUI-64 for autoconfiguration.  The actual IPv6 protocol itself supports all the way down to /127s when it comes to network size.

IE, a particular IPv6 implementation exposes the bit wasting you describe, not the protocol itself.

[Michael Vincent]

There is no such thing as broadcasts in IPv6 - only unicast, multicast and anycast.  So broadcast storms are not a concern on networks.  However, the physical limitations of a contention based media will certainly impose a pragmatic limit the number of acual nodes.  And that limit may be different based on network type - e.g., end users versus a data center server tier.

IPv6 isn't designed to support 2^64 hosts per subnet, per se - although theoretically, if you had 2^64 hosts sitting around, you could get them all addressed on a single IPv6 subnet.  For those of us with long time experience with IPv4 - seeing /24 and even /16 in some cases used on point-to-point links, using IPv6 a single /64 broken in to /126 subnets for point-to-point links will provide more than enough networks to address all your point-to-point links.  On the LAN side however, there are some protocols (ND, RS, SLAAC) that rely on /64, so while you can put the subnet boundary where ever you want, you'll really want to stick with /64.

cheers.

[scottpickett]

Despite the absence of broadcasts in IPv6, the operations of Neighbor Discovery are multicast in nature.  To a switch which does not support multicast, or has multicast features disabled, these multicasts would be flooded like broadcasts.  Likewise, any other application which used IPv6 multicast would also create broadcast like flooding.

Would this not result in identical Layer 2 design requirements (flooding domain size, etc) between IPv4 and IPv6?

[andrecsthompson]

So can I conclude 500?

That's a waste of addresses!

[Peter Koltl]

waste? never mind (-:

http://www.networkworld.com/community/blog/case-127-subnets-1

http://www.networkworld.com/community/blog/logic-bad-ipv6-address-management

"we simply are not going to run out of IPv6 addresses

Go Ahead. Waste a Few Million Trillion Addresses. There's Always More."

I would suggest about 1000 nodes at most in a VLAN. After all, today's CPU's can handle 1000 peers' broadcast multicast packets.