A. Dynamic Versus Static IP Addresses

Here we describe dynamic and static IP addresses, and discuss the advantages and disadvantages associated with each type.

Before connecting your machine to the network, please fill out the Node Registration Form , which allows you to select a DHCP (dynamic) or static IP address. This form also registers your hardware address with the Data Com and Networks group in the Computing Division.

A.1 Dynamic IP (DHCP) Address

Each time a DHCP client boots, it sends out a DHCP discover message. All DHCP servers answer (in practice only one is set to do this at Fermilab; in the future possibly a second will be added for redundancy) with an offer message that includes an address which is available to the client.

The client machine typically repeats the discover message several times to make sure it hears from all the servers, then eventually chooses the "best" server, where what is "best" is up to the client. It may mean that the addresses the DHCP server has available offer the longest lease time. Or the client might prefer a server that provides WINS servers over one that doesn't (the WINS servers keep track of all the clients' and servers' latest dynamic IP addresses).

The currently active DHCP server is configured by hand to handle and reserve IP addresses and the IP configuration information that goes with them. Addresses are made available in an order that permits a client to have the best chance of getting back the same address it was using most recently. To this end, the DHCP server offers its least recently used address to a new client.

Once the client chooses a DHCP server, it "officially" requests the IP address and configuration information. In addition to this, it receives a lease time for the address. This lease time is not absolute. As long as it is running, the client machine requests renewal of the lease. This is invisible to the user, although there is a mechanism for the user to release the address early (ipconfig/release from the command prompt).

Client machines in the NT domain typically access multiple file servers, print servers, and so on. The clients as well as the servers may change their IP addresses. Via the WINS servers, this is transparent to the user.

Advantages

All the IP configuration information gets automatically configured for your client machine by the DHCP server.
If you move your client machine to a different subnet, the client will send out its discover message at boot time and work as usual. However, when you first boot up there you will not be able to get back the IP address you had at your previous location regardless of how little time has passed.

Disadvantage

Your machine name does not change when you get a new IP address. The DNS (Domain Name System) name is associated with your IP address and therefore does change. This only presents a problem if other clients try to access your machine by its DNS name. One example is ftp . If a Windows machine is set up as an ftp server, then its ftp server name (which uses the DNS name) changes every time the IP address does. If you need to use your Windows machine as an ftp server (or as a Web server), request a static IP address rather than a dynamic one.

A.2 Static IP Address

If you have requested a static IP address on the Node Registration Form , you need to wait for the reply with all the information you need to use for configuring your machine. Once you receive it, under Windows, delve down to Start/Settings/Control Panel/Network/Protocols and enter the information that you received.

Advantages

The two names (Windows name and DNS name) are the same as each other, and neither ever changes. Other clients may therefore reliably access your machine by its DNS name (e.g., using ftp ).
With a static address your machine is more easily accessible by non-Windows internet services. This is not a significant advantage as people seldom telnet to their Windows PC.

Disadvantages

You can't move your machine to a different subnet and expect it to work. You need to reconfigure it.
If machines come and go, or are up only some of the time, static assignments are less resource-efficient (where the resource in question is the IP address itself).