Hi all,

How does a router assign dynamic IP addresses?

I have a linksys wireless router with a small home network (2-XP machines, 1-98se machine). I've had this setup for several months with machine IP's assigned as 192.168.1.100, -101 & -102. They have never changed. Even after power outages.

All of sudden, this morning, I have 192.168.1.100, -101 & -103!! Why? What happened to -102? Can I get it back without resorting to assigning STATIC IP's?

It's not a huge problem but I run PCAnywhere and TightVNC from work (outside the network) and when the IP changed I had to reassign port forwarding numbers in the router software.

Again, it's not a big problem but I hate not knowing what's happening. I'm a control freak and it bothers me that I can't change this back (even after powering everything down)!!

Anybody know?

Thanks,

bulloney

DHCP works like this.

A DHCP client broadcasts a request for an IP address configuration lease. The DHCP server sends a configuration lease. Within this lease, there is an expiration time. Your computer will ask for a renewal before the expiration date (if on).

The DHCP server keeps track of which NIC's get which configurations with ip address according to the MAC address of the NIC. It's up to the server when the lease expires if it gives the computer a new IP address or the same one. In this case, it gave your computer a new lease for a new IP address.

To prevent this from happening, some DHCP servers support DHCP reservations. This makes a permanent association of an IP configuration to a MAC address of the NIC receiving the lease, thereby configuring this NIC with the same address everytime. Your router probably supports this. You may need to enter the MAC address for that NIC. If this is an XP machine, start-run-cmd, ipconfig /all to find it. If it's a 98 machine, winipcfg should show it when you select the NIC in the drop down at the top of the window.

MCSE, MCSA Messaging, baby!

Usually you can get it back, by powering off the system and in the router, release or delete its DHCP asignment. Then power it back on so it gets a new one.

To recover from things like a power failure, you can power off all of the systems that have the "wrong" IP address, then power off the router so it resets and then power it back on. Make an Internet request on each of the machines still powered up with the IP address you want them to have to make sure the router picks up that IP address from them. Then power up and boot each of the other systems in the sequence you want them to pick up IP addresses.