PCs
| OS | Use DHCP | Release | Renew |
|---|---|---|---|
| Windows | netsh interface ip set address "Ethernet" dhcp | ipconfig /release | ipconfig /renew |
| Linux | sudo dhclient eth0 | sudo dhclient -r eth0 | sudo dhclient eth0 |
| macOS | sudo ipconfig set en0 DHCP | (System Settings) | sudo ipconfig set en0 DHCP |
A PC that can't reach a DHCP server behaves differently on each OS. Windows and macOS give themselves an APIPA address from 169.254.0.0/16 so they can at least talk on the local link; Linux stays without an IPv4 address.
In NetLab
▶ PC1 NetLab Windows-style command prompt. Type help to see the commands. C:\Users\student>ipconfig /renew Windows IP Configuration An error occurred while renewing interface Ethernet : unable to contact your DHCP server. Request has timed out. C:\Users\student>ipconfig Windows IP Configuration Ethernet adapter Ethernet: Connection-specific DNS Suffix . : Link-local IPv6 Address . . . . . : fe80::11db:99de:fbce:36d1%12 Autoconfiguration IPv4 Address. . : 169.254.141.80 Subnet Mask . . . . . . . . . . . : 255.255.0.0 Default Gateway . . . . . . . . . : ▶ PC2 student@PC2:~$ sudo dhclient eth0 No DHCPOFFERS received. No working leases in persistent database - sleeping.
Remember: 169.254.x.x on a PC means "I asked for DHCP and nobody answered". Check the cable, the switch port's VLAN, the helper address, and the server.
A router as a DHCP client
Internet providers often hand the customer's router its WAN address by DHCP. The interface command is ip address dhcp. The router also installs a default route towards the gateway the server gave it (shown as S* with administrative distance 254).
In NetLab
▶ R1 R1>enable R1#configure terminal Enter configuration commands, one per line. End with CNTL/Z. R1(config)#interface g0/2 R1(config-if)#ip address dhcp *Mar 1 00:00:28.000: %DHCP-6-ADDRESS_ASSIGN: Interface GigabitEthernet0/2 assigned DHCP address 203.0.113.10, mask 255.255.255.0, hostname R1 R1(config-if)#end *Mar 1 00:00:29.000: %SYS-5-CONFIG_I: Configured from console by console R1#show ip interface brief Interface IP-Address OK? Method Status Protocol GigabitEthernet0/0 192.168.10.1 YES manual up up GigabitEthernet0/1 10.0.0.1 YES manual up up GigabitEthernet0/2 203.0.113.10 YES DHCP up up R1#show ip route | begin Gateway Gateway of last resort is 203.0.113.1 to network 0.0.0.0 S* 0.0.0.0/0 [254/0] via 203.0.113.1 10.0.0.0/8 is variably subnetted, 2 subnets, 2 masks C 10.0.0.0/30 is directly connected, GigabitEthernet0/1 L 10.0.0.1/32 is directly connected, GigabitEthernet0/1 192.168.10.0/24 is variably subnetted, 2 subnets, 2 masks C 192.168.10.0/24 is directly connected, GigabitEthernet0/0 L 192.168.10.1/32 is directly connected, GigabitEthernet0/0 203.0.113.0/24 is variably subnetted, 2 subnets, 2 masks C 203.0.113.0/24 is directly connected, GigabitEthernet0/2 L 203.0.113.10/32 is directly connected, GigabitEthernet0/2
Tip: The Method column of
show ip interface briefsaysDHCPfor an address learned this way andmanualfor one you typed.