ipdhcpmodem

How do modems connect with ISP?

AFAIK when i turn on my modem, it says: "Hi ISP, i need an IP". Then, my ISP give it an IP.

  1. How does my ISP identify my modem? by last IP, key, or what?

After i have an IP, i can navigate, but after some hours my modem changes its IP. 2) How is that change done? I mean, my ISP says: "Hi user modem, there is your new IP"

  1. It gets even funnier if when i turn on my modem, some other modem has the last IP my modem had. There is a collision. So, my ISP would give my modem another IP, wont it?

I know im talking about technical stuff, but i would like you to explain me in your own words in order not to make it cumbersome.

If technical references arise, maybe just name the concept or leave a link. It would be enough for me.

Thanks you all!

Solution

  • By posting under the tag „dhcp“, you are obviously already assuming that the DHCP protocol is the answer to your questions:

    1. The DHCP Protocol allows the DHCP Server (your ISP) to identify the DHCP client (your modem) by a multitude of information. The most important one is usually the MAC address of your modem. The last IP is also transmitted from client to server along with proprietary information such as the client identifier and others.

    2. The server supplies the IP address along with a lease time. The client will renew the IP address with the DHCP server when the lease time is about to expire. The server decides in the renewal process triggered by the client if the same IP is ok to use further or not.

    3. Restart is not much different from renewal. The DHCP protocol is for that purpose equipped with a broadcast feature so the collision does not really happen, because the client (modem) will ask for an IP address before it uses the old IP.

    It is possible that modem and ISP do not use DHCP but the mechanism is probably similar. DHCP is specified in RFC 2131.