Dynamic Host Configuration Protocol

aliases:

DHCP

A system that automatically sets and manages network connection configuration settings for incoming hosts.

DHCP removes the need to manually configure the network connection settings of an incoming host, as some do when managing small networks.

Setup

When setting up a DHCP server, you need to define what's known as the scope. The scope is the default network IP configuration sent to incoming hosts.

IP address range (called the scope)
- Can be set to all IP addresses as defined for your network class or be limited to a smaller range within your class' range
Subnet mask
Exclusion & Reservation Addresses (addresses that you DON'T want DHCP to assign to incoming hosts; i.e. reserve them for devices that will be assigned static IP addresses)
- You can also tell your DHCP server to automatically assign a reserved/excluded IP address to a network host according to it's MAC address.
Lease time (how long this specific IP address that we've handed out should remain in the hands of this network host)
- While a lease is active, the IP address cannot and should not be reassigned, even if the device it's leased to disconnects from the network. Halfway through the lease time, the device will attempt to renew it's DHCP lease if it's still connected.
- Depending on the nature of your network, you can set the lease time to a few days (few devices will be coming and going from the network) or even an hour (guest network where devices are indeed coming and going).
Default gateway (where we send stuff that's going outside the local network)
DNS servers (where we translate hostnames to IP addresses for usage)

Depending on the size of your organization, the DHCP server can be ran on the network router or on a small Windows or Linux server.

DHCP Lease Process

Discover (Locate DHCP servers on the network.)
Offer (DHCP server offers the host network config parameters, including an IP address.)
Request (The client accepts the offered IP address.)
- Before sending the Request packet back, the client performs an ARP broadcast on the network, ensuring that there isn't already a host using the offered IP address.
Acknowledge (The DHCP server confirms the IP address lease.)

After 87.5% of the lease on a hosts IP address has elapsed, the host should begin to try to renew it's lease with the DHCP server. This is called the T1 timer.

#XI