Configuration Without Manual Work
When a device joins a network it needs an IP address, a subnet mask, a default gateway, and DNS servers. Setting these by hand on every device does not scale. The Dynamic Host Configuration Protocol, or DHCP, hands them out automatically.
The Four Step Exchange
DHCP uses a four message dance often remembered as discover offer request acknowledge.
- The client broadcasts a discover because it has no address yet.
- One or more servers reply with an offer of an address and settings.
- The client broadcasts a request accepting one specific offer.
- The chosen server sends an acknowledge confirming the lease.
Leases
An address is granted as a lease for a limited time. The client must renew before the lease expires, usually contacting the server at the halfway point. If it leaves the network, the address eventually returns to the pool for reuse.
What Else It Delivers
Beyond an address, DHCP supplies the subnet mask, the default gateway for off network traffic, and the DNS servers to resolve names. This is why a laptop can join a new network and reach the internet with no manual setup.
Key idea
DHCP leases an IP address and network settings through a discover offer request acknowledge exchange so devices configure themselves automatically when they join a network.