Network Utilities

A Guide to Understanding DHCP

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DHCP stands for Dynamic Host Control Protocol. It does one thing for you. It assigns an ip address, subnet, and gateway to each computer on your local area network. If you don't know the terms I just refered to you can visit our help page. I'll try to briefly go over what an ip address is here, because you need to understand that term to understand dhcp. You don't really have to understand what a subnet or gateway is to understand dhcp. It is enough to know that the subnet and gateway can be handed out via dhcp.

An IP Address is a unique number given to every computer on a network. Once again I say unique, because two computers on the same network can not have the same IP address. The ip address generally takes for form of four 0 through 255 numbers which are seperated by periods. A common ip address would be 192.168.1.2.

Everytime a computer is turned on it contacts the dhcp server to request an ip address, subnet and gateway. The dhcp server generally provides the next availble ip address. This is more important than it sounds. The order in which the computers are turned on effect which computer gets which ip address. One day you turn computer A on first and it gets the 192.168.1.2 ip address. The next day you turn computer A is turned on second and it gets a 192.168.1.3 ip address. Port Forwarding forwards a port to a certain ip address. If that IP address changed, then that port forwarding configuration would not be setup properly. One day your port forwarding configuration would work. The day it might not. In order to avoid this problem, we need to setup a static ip address on the computer that the ports are going to be forwarded to.

A static ip address is an ip address that is not assigned by DHCP. A static ip address is an ip address that has been manually setup on that computer. DHCP actually knows nothing about an ip address that has been assigned staticly. It doesn't even know that ip address has been assigned. So it can very easily hand out an ip address via dhcp that is the same as the one that has been assigned statically. At the start of this explaination, we said that an ip address needs to be unique. When two computers have the same ip address there is an ip address conflict and neither of them can connect to the internet. Let's go back to computer A and computer B. Computer A has the static ip address 192.168.1.2. Computer B boots onto the network and DHCP then hands it the first available ip address. Which it considers to be 192.168.1.2.

There are a couple really easy ways arround this. The first one is to just assign an ip address that is far away from the dhcp assigned ip address. If dhcp assigns 192.168.1.2 as it's first assignable ip address, you should assign a static ip address that is something like 192.168.1.50 or more. It's pretty unlikely that you will have 50 computers on your network, so there will never be an ip address conflict. If the first ip address handed out by dhcp was something like 192.168.1.253, you should enter should assign an ip address that is 192.168.1.150 or less. The only thing that matters is that there are no ip address conflicts. Just make sure you do not setup an static ip address that is one of the ip address that dhcp normally hands out.

The second and safest way to avoid ip address conflicts it to modify the range of ip addresses that dhcp hands out. We have written guides to do this for each particular router. Well we wrote guides for all the routers we had screenshots of. This method is really simple. We tell the DHCP server to only hand out certain ip addresses. Then we assign a static ip address that is not being handed out by dhcp. Here is an example. We tell the dhcp server to hand out the ip addresses from 192.168.1.2 to 192.168.1.10. Then we assign a static ip address to computer A that is 192.168.1.11. Since the DHCP server will never hand out 192.168.1.11, there will never be an ip address conflict. Any ip address that is higher than 192.168.1.10 would work as a static ip address.

Jason Bauer

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Jason Bauer is an owner and programmer for Portforward.com. You can find more of his articles here.