Dynamic Host Configuration Protocol * Automatically assigns an IP address whenever a device connects to the network.

DHCP four-way handshake

DORA Discover Offer Request Acknowledgement

Ports/Protocols

Uses UDP ports 67 and 68 * Sending: 67 * Receiving: 68

DHCP relays

On larger networks with multiple LANs sometimes it's necessary to relay DHCP broadcasts through to the central DHCP server. Routers block DHCP broadcasts by default.

IP reservation

Can configure DHCP servers to reserve certain IPs in use as static IPs on some machines

MAC Reservations

Rarely used, but sometimes it makes more sense to have the same MAC address always associated with the same IP.

No DHCP Server

Zeroconf

Troubleshooting

1. Try to re-establish lease manually Windows: ipconfig /release ipconfig /renew Mac: sudo ifconfig eth0 down sudo ifconfig eth0 up Linux: sudo dhclient -r sudo dhclient

Multiple DHCP Servers

Usually in a failover configuration.

Rogue DHCP Server

A DHCP server is added to the network and is giving out incorrect IP addresses. Often this is either a user brings in a router from home and plugs it into the network or it's a malicious actor that has gained access to the network.