Let's just plug this baby in...
Here's a typical scenario:
- A student has a residential gateway / router at home that typically includes an integrated multi-port switch (with Uplink capability) and sometimes built-in Wireless capability.
- The student brings the router to campus and connects the Uplink port to the campus network so they can attach multiple computers to the campus network, and perhaps use the router's wireless capability for a wireless-equipped laptop. The router configuration is usually not changed from the home settings, where the router was used to share a DSL or cable modem connection.
The main problem comes from the fact that since the LAN side of the router is now connected to the campus network, the router's DHCP server will try to hand out DHCP leases to new computers as they are attached to the network and request IP address information. Normally, these requests would be answered by the campus DHCP server, which also hands out Gateway and DNS information - essential for proper Internet connection - along with an IP address and subnet mask appropriate to the subnet that the computer has been connected to. But since the "rogue" DHCP servers also hear the request and may not be as busy as the campus DHCP server, the "rogues" often win the race and answer the DHCP request first.
The result of this DHCP server battle is that the requesting computer not only ends up with an incorrect IP address that is based on the router's DHCP server set up, but it also gets incorrect Gateway and DNS information. (That information would have been obtained by the router's WAN-side DHCP client... if the WAN port were connected.) This yields a throughly confused computer that might be able to communicate with some other machines (depending on where they got their IP address info from), but that certainly can't connect to the Internet due to the incorrect Gateway and DNS info.
On the other hand, some computers may not get a DHCP lease at all if the campus DHCP server never gets to answer the request. This is bcasue most consumer routers are configured to hand out only a few dozen addresses. Once the limited number of leases has been exhausted, the server will report "out of addresses" to the next requesters