Patent ID: 6865611

Claim:
A communications system comprising a plurality of top-level domains each having a respective top-level address space; a next-level domain connected one of directly and indirectly to more than one of the plurality of top-level domains, the next-level domain having respective next-level address spaces which are subsets of the respective top-level address spaces of the more than one of the plurality of top-level domains; and a host connected to the next-level domain, the host having host addresses which are members of the respective next-level address spaces such that, when a route is available from any of the more than one of the plurality of top-level domains to which the next-level domain is connected, the next-level domain is arranged to receive information from the top-level domains having an available route, and the host is arranged to use the information to select, from the host addresses, for use as its source address on transmission, an address corresponding to a route which is available.