[xxpor]

If they're not actually using the whole /8 (highly likely), you can setup a 1:1 NAT. basically from network b, if you want to talk to network a, you find out the address in 11/8 that corresponds to the 10/8 address and vice versa. You can use split horizon dns to make it mostly transparent.

Every networking problem in the world can be solved with more NAT or more encapsulation :)

[jandrese]

You don't have to use every address in 10.0.0.0/8 to effectively fill it up. If your corporate policy is to assign a /16 to each floor of a building, and you have a LOT of buildings it's pretty easy to fill up the space even if most of the /16s are sparsely populated. It's much easier to move on to the 11. space when you build that new building that pushes you over than renumbering your entire corporate LAN.

[xxpor]

Right, but that's not relevant for 1:1 NAT (well, at least it doesn't have to be). Since the NAT would happen in software, you're no longer constrained by subnets being physically under routers. 11.2.3.0/24 could contain 10.0.1.0/24 and 10.128.128.0/24 without any issues, assuming they don't use in total more than 256 address.

[woleium]

what you call 1:1 NAT is just called NAT by cisco, the stuff most folks think NAT is is actually NAT+PAT (like what you run on your home router with a single public IP)

[Dylan16807]

Mostly, but NAT without PAT doesn't have to be 1:1. You could put 100 hosts behind 20 IP addresses without ever changing a port.

[xxpor]

exactly, that's why I specified.