Thousands of hosts on a gigabit LAN, maybe not

Fri May 8 19:54:16 UTC 2015

Linux has a (configurable) limit on the neighbor table. I know in RHEL 
variants, the default has been 1024 neighbors for a while.

net.ipv4.neigh.default.gc_thresh3
net.ipv4.neigh.default.gc_thresh2
net.ipv4.neigh.default.gc_thresh1

net.ipv6.neigh.default.gc_thresh3
net.ipv6.neigh.default.gc_thresh2
net.ipv6.neigh.default.gc_thresh1

These may be rough guidelines for performance or arbitrary limits 
someone thought would be a good idea. Either way, you'll need to 
increase the number if you're using IP on Linux.

Although not explicitly stated, I would assume that these computers may 
be virtualized or inside some sort of blade chassis (which reduces the 
number of physical cables to a switch). Strictly speaking, I see no 
hardware limitation in your way, as most top of rack switches will 
easily do a few thousand or 10's of thousands of MAC entries and a few 
thousand hosts can fit inside a single IP4 or IP6 subnet. There are some 
pretty dense switches if you actually do need 1000 ports, but as others 
have stated, you'll utilize a good portion of the rack in cable and 
connectors.

--Blake