Ok - now one from my experience...
I used to work for s mall company in downtown New York.
Owner of that company had a friend (frenemy?) on the same floor (owner of accounting firm). After I (and another very good SysAdmin) left the left the firm, said owner got some $90/h Ms Cert Partner consulting to take care of the network and systems.
The internet setup we left behind was public IP network (remember - it's 2000) on a cisco router with partial T3.
So, owner decides to get back some T3 cost and sell internet connectivity to accountant for $50/month. Consulting firm handles the detail. For a while everything is great! Things work. But all of the sudden random PCs in both offices can't access network (LAN or WAN). I just happened to stop by - they beg me to look at it. So, here is the multiple FAIL:
- consulting firm just ran the cable from available Cisco interface to accountant's office: no NAT, no routing, basically just combined two layer 2 networks (a major no-no, but seemed to work for a while)
- accountant decides at some point it'll be good to sublet a room to an attorney.
- said attorney's "tech savvy" partner sets up a network with server (but it's own private IP subnet) on the same switch as accountant's network.
- things would still work and noone would know how f%$#d up the setup is if said "tech savvy" partner didn't setup DHCP server.
So, 2 DHCP servers on the same LAN (same vlan). PCs were just getting private IPs from attorney's server.
I told them thir options:
1. Cut the freaking L2 uplink.
2. Pay me same ($90/h that idiot from consulting firm charged) to setup NAT and such.
They opted for cut uplink, cheap bastards. :-P