What happens to network when DHCP server disappears?

Factual Questions
1

What happens on a small Windows peer-to-peer network that is set up to get IP addresses automatically, if the DHCP host disappears?

This is a long story, posted elsewhere, but basically I may have to configure every PC and appliance on my home network to be a DHCP client, because STUPID DIRECWAY requires you to do that to use their service. Trouble is, their “modem” is in a remote location, and I am afraid the rest of the network may not have reliable access to it. So I expect all my equipment to become unavailable whenever the modem is unavailable.

Is there anything I am missing about this? For example, is there a program that can keep looking for a DHCP server, and do nothing if it finds one, but when it can’t find one it would become the DHCP server itself? Then, when another device tries to be the DHCP server again, this program would figure that out and stop serving?

Can you have multiple DHCP servers in the same network?

2

Use a router to connect to the DHCP host. The router itself can act as an internal DHPC server (or there are other configurations for larger networks). If the external DHCP fails that has nothing to do with the internal network configuation.

But, I am not sure what would happen in your scenario.

3

You CANNOT have 2 DHCP servers on the same network serving into the same address space. that’s arecipe for disaster.

BUT, the one DHCP server you do need does NOT have to be continuously available.

DHCP operates on a lease basis, where when a machine first boots without an address it broadcasts a request for any server to give it one. When it gets an address it also gets a lease expiration or drop dead time. The client machinewill use that given IP address until that time, even if the client is rebooted in the meantime. So it does NOT need to renew the address with the DHCP server every time it boots up, nor very often.

Once it gets halfway through the life of the lease, the client will start trying to renew the lease.

Finally, if the lease expires, the machine will continue to use the address it was given, whiel searching for somebody / anybody to give it a new lease.

For a home network with very little day to day change in the roster of machines, you can run for a long time without any DHCP server at, once you’vve had one working to give everyone their initial lease.

On my home network I use a DLink 704 firewall/router wich does DHCP serving. The lease duration is not configurable, but is apparently 41 days.

To see the lease duration of your current DHCP server, open up a command window & type >ipconfig /all. The last two lines give you the timestamps for lease obtained and lease expired, and you can subtract to see the duration.

Bottom line, as long as your DHCP server is available more often than 1/2 it’s lease duration you’re fine. And you DON’T want two DHCP servers on a network simultaneosly, or taking turns. Backup DHCP servers are common in commercial environments, but they’re software systems that are talking to each other to stay in sync. Two hardware devices from different manufacturers taking uncoordinated turns at being the sole DHCP server would be BAD.

4

This is only true if the machines are not rebooted or otherwise forced to lose the current configuration. For example, if you restart the system, the previous setup will be lost and it will look for a DHCP server. Not finding one, the machine will self-configure with an address from the 169.254.0.0 DHCP autoconfiguration address space. To demonstrate this, disconnect your machine from the network and reboot, then check the networking configuration. The IP address will be 169.254.x.x, the netmask 255.255.0.0, and there will be no router or name server configured.

I would suggest, as erislover recommended, the purchase of a NAT router/gateway that recieves its IP address from the remote modem, and poesseses a local DHCP server to serve the machines. This also has the added benefit of protecting your computers against attacks from the Internet.