10-21-2009, 12:56 AM
Your cable modem acts as a DHCP server to your router - it provides the IP address the router uses to communicate with the modem. Likewise, your router acts as a DHCP server to your VOIP server and other downstream network devices, with some NAT translation thrown in to enable you to utilize private IP addresses for your LAN.
If the DHCP server that provides an IP address to a device is not active when the device attempts to come online, the device will not be able to connect to the network. Often the device will assign itself a 169.254.x.x IP address, which will preclude it from joining the network once the DHCP server DOES come online until the network device is rebooted (for more info see http://support.apple.com/kb/TA26003).
(Basically what Racer X said, with more technical detail provided.)
If the DHCP server that provides an IP address to a device is not active when the device attempts to come online, the device will not be able to connect to the network. Often the device will assign itself a 169.254.x.x IP address, which will preclude it from joining the network once the DHCP server DOES come online until the network device is rebooted (for more info see http://support.apple.com/kb/TA26003).
(Basically what Racer X said, with more technical detail provided.)