Forcing a Cable Modem to get a new IP

[Marc P.]

Cable modems have their own built in MAC address, besides passing on the MAC of the associated computer or router. That's how cable companies keep track of who is using their network (unlike DSL which relies on authentication via PPPoE.)

Sanddrag, DHCP servers assign IP addresses with a set lease time, sometimes 3 days, sometimes 7 days, depending on how the network is set up. Most of the time, the release and renewal when an IP expires is transparent, it happens in milliseconds and there's rarely a disruption of service. The problem you're having is most DHCP servers cache MAC addresses in a client table, and tend to assign the same IP associated with a given MAC address each renewal. Network operators and their servers appreciate consistency, and on a network with potentially thousands of clients, the more consistent addresses are, the less stress there is on the server to straighten out the client table and make sure everyone gets an address. Rather than try finding the next available address for your system, it remembers you had that address last renewal, and gives it back to you.

Only way around that (short of calling your ISP directly and asking if they can clear your MAC out of the client table) is to find out how long your IP lease is, and unplug your cable modem an hour before the lease expires, and plug it back in an hour after the lease expires. If your modem isn't online to negotiate the renewal, the DHCP server should chop it out of the client table and assign the IP to another client. Next time your modem communicates with the DHCP server, it should assign the next available IP in it's block. Generally, the longer you wait after the expiration, the better chance you'll have to get a new IP (the assumption is another client will come online to take your old IP before you do).