If you have a DHCP address your machine will periodically attempt to renew the address from the server it was leased from.

The renewal happens when the DHCP client starts, or at half the lease period. IIRC it will contact the server directly, so a client with a reasonable lease time (say 30 days) should never actually lose it's lease as each time it is renewed it will get another 30 days.

You will only get problems if your lease is revoked or expired. This can happen when:
  • It is manually revoked on the server
  • It is manually released by the client (ipconfig /release)
  • The DHCP server is not contactable when the client tries top renew it at the lease half-life, or the client is switched off.
Once the lease is gone (or if this is a client which has no lease) you will get problems.
This is because the DHCP request is a local broadcast - until it gets the DHCP information the client will not have any routing/gateway information, and will not in any case know what the IP addres of the DHCP server is.

As it is a local broadcast it will not be propogated across the WAN - unless you are operating in a bridged mode.

Your router/concentrator will see the broadcast, and relay or proxy the request directly to the DHCP server. It will add information about the LAN address that the request was relayed from, which will allow the DHCP server to identify which address pool to allocate the IP address from. The response packet is send back to the router, which directs it to the client.

Imagine trying to get some advice about a leak from a plumber at the other end of the 'phone. You cannot use the phone because you are too far away from it and are anyway holding on to the leaky pipe. Your long suffering partner (who told you to call a professional) relays the information across the 'phone.

No matter how loud or how frequently you shout you will never hear an answer - you need the relay to do it for you.