blocked LAN ip address

i have a imac (snow leopard) as the database server for a POS system. it also is used as the main register for the POS. a second mac (mini - mountain lion) was used, sometimes as a register, sometimes for POS maintenance (inventory, etc.). at some point the ML mini refused to 'see' the DB server; i couldn't establish any communication between the two, even tho' the ML mini could still access the internet, and LAN peripherals (remote speakers, network printer, NAS backup drive).

this communication failure only applied the to ML mini. there are another mini and another imac running snow leopard, and a macbook running mountain lion; all can run the POS user software and connect to the DB server without problems.

the LAN is handled by an newish airport extreme, with the DB server having a reserved ip address. i changed the reserved address for the DB server, and that solved the problem. except that 2 months later, the blockage occurred on the ML mini. this time i swapped it with the snow leopard mini before changing the DB server's ip reservation. problem solved (again) with all macs being able to connect to DB server. the swap was only their physical locations in the store; the entire network is wireless.

now the snow leopard mini is having the same problem (tho' this time the mountain lion mini is not). snow leopard mini has all LAN/WAN access except for DB server.

ao i'm thinking it's the DB server mac, not the POS clients. but either way, i'm more windows-based and only mildly network-literate, so i don't know where to look on a mac to see why it would decide to block an ip address, nor how to flush/reset/??? such configuration should i find it. i can keep changing the DB server's ip reservation every month or so, but i would prefer to find the underlying problem and correct it once and for all.

any suggestions on where to start looking? (or if this is not the right community which might be?)