Dynamic Host Configuration Procotol
From TSG Library of Knowledge
A Dynamic Host Configuration Protocol (DHCP) server is a device on the network that assumes the role of handing out network information and assigns IP addresses to a networkable device that asks for it.
There are a couple common types of hardware that has this function: a computer with server responsibilities, and routers.
How does it work
First the DHCP server will need to have a scope configured. A scope (also known as a pool) is a range of IP addresses that is determined by the network administrator. The network administrator can then exclude certain IP addresses to allow other devices that require static IP addresses. This way this ensures that the DHCP server does not mistakenly give out an IP address that is already in use. If two machines have the same IP address, then there will be a conflict and one or both devices will not work on the network until it gets resolved. The network administrator would then also configure other key network information listed below.
Next, it will wait until it hears a networkable device looking for a DHCP server. This is known as discovery. When the DHCP hears the discovery, it will let the device know it is there and will give it an IP address. This is known as an offer. When it hears the 'offer', the device will ask for an IP address, known as request. Lastly the DHCP server will give the information to the device. This is known as acknowledgement. In a conversation, it would look something like this:
PC: Is there anyone out there to give me the network information? (discovery)
DHCP: I can give you an IP address! (offer)
PC: Sweet! Can I have it please? (request)
DHCP: No problem! Here are your details... (acknowledgement)
When this transaction occurs, the DHCP server will give the device a expiry date for the IP address. This is also known as a lease. A lease is basically a time limit on how long does a device keeps the IP address. Once it gets near to the expiry time limit, the DHCP will check with the device if it's still there. If the device does not return back to the DHCP in time, the DHCP will 'release' the IP address and then puts it back into the DHCP pool for other machines to use.
One other way a device can get an IP address is via a reservation. A network administrator can reserve an IP address for a device via their MAC Address, so when a device with a specific MAC Address asks for an IP address, it would usually get the IP address that was assigned by the network administrator.
More than just an IP address?
The DHCP server not only assigns IP addresses, but it also gives out other key network information: a subnet mask, a default gateway and DNS server information. THis information is configured by the network administrator. You can find this information by going to Start, Run, type ipconfig /all. This command provides you with detailed information about your Network Interface Card (NIC), which includes the key information, including the lease time.
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