this is already implemeted in the gnutella v0.4 protocal, and you can even do this with some programs like limewire (i think),

when any computer makes a connection request to a host on the network, the first thing it does once the connection is accepted is send the string "GNUTELLA CONNECT/<protocal version string>\n\n"
protocal version string usually = "0.4"

then, if the recieving host wishes to mantain the connection, it must reply with "GNUTELLA OK\n\n" if it doesn't, the connection is terminated

if you want an 80's subnetwork, all you have to do is change the connection string (again, i think limewire lets you do this)

the only thing that needs to be done is to standardize the 80's connection string and provide IP numbers to computers in the subnetwork with a host cach server