Why would a local network unit's ip number change?

[Eric Shubert]

joe at actionline.com wrote:
> I had rsync working fine between two computers on my local network, but
> then it just quit working even though I had made no changes in either
> computer.
> 
> This is the error message I saw:
> ssh: connect to host 192.168.0.73 port 22: No route to host
> rsync: connection unexpectedly closed (0 bytes received so far) [sender]
> rsync error: error in rsync protocol data stream (code 12) at io.c(601)
> [sender=3.0.7]
> ssh: connect to host 192.168.0.73 port 22: No route to host
> rsync: connection unexpectedly closed (0 bytes received so far) [sender]
> rsync error: unexplained error (code 255) at io.c(601) [sender=3.0.7]
> 
> Upon investigating, I found that the ip number on the "73" computer was
> changed from 192.168.0.73 to 192.168.1.106.  I tried changing the
> destination address to "1.106" on the sending computer and that also did
> not work.  So I rebooted the destination computer and found that the ip
> number was restored to "0.73" again.
> 
> What would cause the ip number to have changed like that and how can I
> prevent that from occurring again in the future?
> 
> 
> 

You need to understand how the host's IP address is assigned. Since it's 
changing, it's fair to say that DHCP is being used. DHCP is a mechanism 
where a DHCP server gives networking information (IP address, gateway, 
DNS addresses) to hosts on the subnet that it serves (your LAN). Since 
you have a private (192.168.x.x) address, it's fair to say that you have 
a DHCP server in your network appliance, ie DSL modem or wireless router.

The way DHCP works is that it assigns IP addresses from a pool (range) 
of addresses within the subnet. The IP address it gives to a given host 
(MAC address technically) does not typically change, but it can. What's 
a little disturbing about the addresses you've received is that they're 
from two different subnets (one is 192.168.0.x, and the other is 
192.168.1.x, which are usually different subnets).

Do you have multiple networking appliances hooked up? I'm guessing that 
you have 2 different DHCP servers operating on your lan, one on a DSL 
modem and another on a wireless router. Your computer could acquire a 
different address, based on whichever DHCP server replied first to your 
computer's request for an address.

So what does your network configuration look like? What devices do you 
have, and how are they connected?

-- 
-Eric 'shubes'