I wonder if it would suit into Zabbix to have the option for defining networks.
These networks could maybe defined by IP address + Netmask (optionally + Tag + Site) and may provide further fields whose values are all available via macros.
Every host interface could be automatically mapped against a network if it exists.

Beside messages, commands, etc. the networks could then be used for selection similar to host groups or wherever else useful (e.g. for network discovery).
Having the option to add them automatically via auto-registration could be interesting too.

The idea for this issue is driven by http://www.gestioip.net/
I don't want Zabbix to become a network management solution like GestióIP but I imagine that it would be beneficial to be able to define networks with some information and put them into relation to hosts.