Network Address Translation:
Cisco – IP NAT and PAT Examples:

Example 1:

• Allow all outgoing traffic
• PAT local network as 100.100.100.100
• Allow incoming SMTP traffic (TCP Port 25) to 192.168.1.5 that will be NAT’d as 100.100.100.101
• Default Gateway 100.100.100.102

!
Interface Ethernet0
IP address 100.100.100.101 255.255.255.248
Description Outside-NIC
IP nat outside
IP virtual-reassembly

Interface FastEthernet0
IP address 192.168.1.254 255.255.255.0
Description Inside-NIC
IP NAT inside
IP virtual-reassembly

IP route 0.0.0.0 0.0.0.0 100.100.100.102

IP nat pool nat-1 100.100.100.100 100.100.100.100 netmask 255.255.255.248
IP nat inside source list 109 pool nat-1 overload
IP nat inside source static tcp 192.168.1.5 25 100.100.100.101 25 extendable no-alias

access-list 109 permit IP 192.168.1.0 0.0.0.255 any

Example 2:
Example of Sub-Interface:

interface Serial0
no ip address
ip nat outside
ip virtual-reassembly
encapsulation frame-relay IETF
no fair-queue
service-module t1 timeslots 1-24
frame-relay lmi-type ansi

interface Serial0.1 point-to-point
ip address 100.100.100.1 255.255.255.252
ip access-group 109 in
ip nat outside
ip virtual-reassembly
frame-relay interface-dlci 500 IETF
crypto map myvpn

If you find this article helpful, please send us a note to Mike@bostonIT.com so I can keep on adding quality hands-on articles.