Configuring NAT Overload On A Cisco Router

Introduction

NAT (Network Address Translation) is a method that allows the translation (modification) of IP addresses while packets/datagrams are traversing the network. NAT Overload, also known as PAT (Port Address Translation) is essentially NAT with the added feature of TCP/UDP ports translation.

The main purpose of NAT is to hide the IP address (usually private) of a client in order to reserve the public address space. For example a complete network with 100 hosts can have 100 private IP addresses and still be visible to the outside world (internet) as a single IP address. Other benefits of NAT include security and economical usage of the IP address ranges at hand.

The following steps explain basic Cisco router NAT Overload configuration. NAT overload is the most common operation in most businesses around the world, as it enables the whole network to access the Internet using one single real IP address. If you would like to know more about the NAT theory, be sure to read our popular NAT articles, which explain in great depth the NAT functions and applications in today's networks.

Switch virtual interface SVI configuration on gns3

A switch virtual interface (SVI) is a VLAN of switch ports represented by one interface to a routing or bridging system. There is no physical interface for the VLAN and the SVI provides the Layer 3 processing for packets from all switch ports associated with the VLAN.

How Switch virtual interface SVI  works:

This is much simpler than inter VLAN, but disadvantage is high cost of a layer-3 switch.

In SVI configuration we need a layer-3 switch, I use a 3640 router IOS and use the NM-16ESW module for layer-3 switching. The topology I used for switch virtual interface (SVI) is consist of one switch and two hosts one of them is in VLAN 10 and other is in VLAN 20. I have create two switch virtual interface s(SVI) on switch for VLAN 10 and 20, which are responsible for communicating these two VLANs.

 

Configuration for Switch:

SW#vlan database

SW(vlan)#vlan 10 name office1

SW(vlan)#vlan 20 name office2

SW(vlan)#exit

SW(config-if)#interface vlan 10

SW(config-if)#ip address 10.1.1.1 255.255.255.0

SW(config-if)#no shu

SW(config-if)#interface vlan 20

SW(config-if)#ip address 20.1.1.1 255.255.255.0

SW(config-if)#exit

SW(config)#ip routing

SW(config-if)#interface f0/0

SW(config-if)#no shutdown

SW(config-if)# switchport mode access

SW(config-if)# switchport access vlan 10

SW(config-if)#interface f0/1

SW(config-if)#no shutdown

SW(config-if)#switchport mode access

SW(config-if)# switchport access vlan 20

OSPF single area configuration Gns3 Lab:

This is tutorial for OSPF(back bone area) configuration on gns3. Connect the router as shown in figure and complete the configuration as under. You can configure the OSPF on Cisco router by two ways:

1.       By network command under the OSPF process

2.       By “ip ospf “commands under the desire interface (support by only new IOS versions)

I have configured R1 & R2 by by method one and other two by “ip ospf” command.

 

R1 config:

R1(config)# interface Serial1/0

R1(config-if)# ip address 172.16.1.1 255.255.0.0

R1(config-if)#no shut

R1(config-if)#exit

R1(config)# router ospf 1

R1(config-router)#Router-id 1.1.1.1

R1(config-router)#network 172.16.0.0 0.0.255.255 area 0

R5 config:

R2(config)# interface Serial1/0

R2(config-if)#  ip address 172.16.1.2 255.255.0.0

R2(config-if)#  no shut

R2(config-if)#  interface FastEthernet2/0

 R2(config-if)#  ip address 10.1.1.1 255.255.255.0

R2(config-if)#  no shut

R2(config)# router ospf 1

R2(config-router)#Router-id 2.2.2.2

R2(config-router)#network 172.16.0.0 0.0.255.255 area 0

R2(config-router)#network 10.0.0 0.255.255.255 area 0

R4 config:

R3(config)# interface FastEthernet1/1

R3(config-if)#  ip address 12.1.1.2 255.255.255.0

 R3(config-if)# ip ospf 1 area 0

R3(config-if)#  no shut

R3(config)# interface FastEthernet2/0

 R3(config-if)# ip address 10.1.1.2 255.255.255.0

 R3(config)# ip ospf 1 area 0

R3(config-if)#  no shut

R3(config)# router ospf 1

R3(config-router)# Router-id 3.3.3.3

R3 config:

R4(config)# interface FastEthernet1/0

 R4(config-if)# ip address 12.1.1.1 255.255.255.0

 R4(config-if)# ip ospf 1 area 0

R4(config-if)#  no shut

R4(config)# router ospf 1

R4(config-router)# Router-id 4.4.4.4

Verifications:

1.       Use the ”show ip route” you will find the three OSPF routes on each router.

2.       End to end ping should be successful.