UDP 67 (server) / 68 (client). IPv4 clients start with a broadcast DORA: Discover → Offer → Request → Ack. Renew unicast at 50 % of the lease, broadcast at 87.5 %, full DORA when expired.

Relay — the cross-subnet piece

Broadcasts die at routers, so the first L3 hop relays: ip helper-address 192.168.100.10 converts broadcast → unicast and stamps GIADDR (its own interface IP) so the server picks the matching pool. helper also forwards TFTP, DNS, TACACS+, NetBIOS.

ip dhcp excluded-address 192.168.10.1 192.168.10.20
ip dhcp pool VLAN10
 network 192.168.10.0 255.255.255.0
 default-router 192.168.10.1   ! option 3
 dns-server 8.8.8.8            ! option 6
 option 150 ip 192.168.100.5   ! IP-phone TFTP (Cisco)

Options worth knowing: 3 gateway, 6 DNS, 43 vendor-specific (WLC for APs), 60 vendor class, 66/67 bootfile, 82 relay-agent info.

Failures, ranked: missing helper, wrong VLAN, pool doesn’t match GIADDR subnet, snooping blocking the offers. show ip dhcp binding / pool / server statistics.