This entry will show a basic working configuration for a Cisco router. This configuration is from a Cisco 1841 running IOS 12.4(3h). The complete configuration is at the bottom of this post. The configuration uses DHCP on the WAN interface so the router will obtain an IP address from the ISP. The configuration uses NAT/PAT to allow multiple computer and devices to get online. The router will act as a DHCP server for the LAN.
DHCP For LAN
ip dhcp excluded-address 192.168.70.1 192.168.70.99 ip dhcp excluded-address 192.168.70.110 192.168.70.254 ! ip dhcp pool LAN network 192.168.70.0 255.255.255.0 dns-server 126.96.36.199 188.8.131.52 default-router 192.168.70.1 lease 8
FastEthernet0/1 is setup as our LAN interface and has the IP address 192.168.70.1. Here we are enabling DHCP for the 192.168.70.0/24 network. The router will now hand out all IP addresses on that network. Up above we exclude 192.168.70.1 through 192.168.70.99. Then we exclude 192.168.70.110 through 192.168.70.254. This limits our IP pool to 10 IP addresses: 192.168.70.100 through 192.168.70.109. The DHCP server will provide 184.108.40.206 and 220.127.116.11 as two DNS servers for the client to use. 192.168.70.1 is provided as the default gateway. lease 8 specifies that the lease lasts for 8 days. If you omit that it will default to 1 day. lease is specified in day hour minute. If you want the lease to last for 8 hours and 30 minutes you would do:
stupiderror(dhcp-config)#lease 0 8 30
Cisco calls this PAT (port address translation) pretty much everyone else refers to it as NAT (network address translation). This configuration will replace any LAN IP address that is going to the Internet with the IP address on FastEthernet0/0, which is acting as our WAN interface.
interface FastEthernet0/0 description WAN Interface ip address dhcp ip nat outside ! interface FastEthernet0/1 description LAN Interface ip address 192.168.70.1 255.255.255.0 ip nat inside ! ip nat inside source list 1 interface FastEthernet0/0 overload ! access-list 1 permit 192.168.70.0 0.0.0.255
On FastEthernet0/0 we have the command ip nat outside specifying it as the outside interface. On FastEthernet0/1 we have a similar command that specifies inside. We also have an access list that permits 192.168.70.0/24. Those three commands won’t do anything on their own. ip nat inside source list 1 interface FastEthernet0/0 overload is what ties everything together. This says on the inside interface apply source access-list 1 and overload it on interface FastEthernet0/0. We need to overload it because we can have up to 254 hosts on the LAN and we only have one IP address on the WAN (FastEthernet0/0). If a host were to get on the network that had an IP address outside of 192.168.70.0/24 and was somehow able to reach FastEthernet0/1 (would pretty much require a secondary IP address on the interface) the access-list would prevent it from being translated if it were to exit FastEthernet0/0.
stupiderror#sh run Building configuration... *Feb 21 14:35:18.419: %SYS-5-CONFIG_I: Configured from console by console Current configuration : 1439 bytes ! version 12.4 service timestamps debug datetime msec service timestamps log datetime msec service password-encryption ! hostname stupiderror ! boot-start-marker boot-end-marker ! enable secret 5 $1$4D28$g74YPSLPm4lhTQR2Iuy01. ! no aaa new-model ! resource policy ! mmi polling-interval 60 no mmi auto-configure no mmi pvc mmi snmp-timeout 180 ip subnet-zero ip cef ! ! no ip dhcp use vrf connected ip dhcp excluded-address 192.168.70.1 192.168.70.99 ip dhcp excluded-address 192.168.70.110 192.168.70.254 ! ip dhcp pool LAN network 192.168.70.0 255.255.255.0 dns-server 18.104.22.168 22.214.171.124 default-router 192.168.70.1 lease 8 ! ! no ip domain lookup ip domain name example.net ip ssh version 2 ! ! ! ! username stupiderror privilege 15 secret 5 $1$BG7S$BAvySovWUZ4k58nfOKXSu9 ! ! ! ! ! interface FastEthernet0/0 description WAN Interface ip address dhcp ip nat outside ip virtual-reassembly duplex auto speed auto ! interface FastEthernet0/1 description LAN Interface ip address 192.168.70.1 255.255.255.0 ip nat inside ip virtual-reassembly duplex auto speed auto ! interface Serial0/0/0 no ip address shutdown ! interface Serial0/1/0 no ip address shutdown ! ip classless ! ip http server no ip http secure-server ip nat inside source list 1 interface FastEthernet0/0 overload ! access-list 1 permit 192.168.70.0 0.0.0.255 ! ! control-plane ! ! line con 0 line aux 0 line vty 0 4 login local transport input ssh line vty 5 15 login local transport input ssh ! end