NAT Cisco IOS

Network Address Translantion

For an explanation of NAT see Wikipedias Network address translation

Cisco NAT

Static NAT

In static NAT The Internal IP address is always translated to the same External IP address, on a one-to-one basis.

Example of static NAT

In the example below, a company has acquired an Internet connection with four additional addresses 195.181.54.0/29.

Connection Link address to internet 83.90.42.0/30
- ISP uses 83.90.42.2/30
- Company Router 83.90.42.1/30
The network 195.181.54.0/29 is routed to 83.90.42.1 by the ISP
195.181.54.1/29 is a real IP address the company wants to use to their WEB-server
- The companys WEB-server is located on the internal private network on local IP Address 192.168.22.110
195.181.54.2/29 is a real IP address the company wants to use to their MAIL-server
- The companys MAIL-server is located on the internal private network on local IP Address 192.168.22.111

Cisco Static NAT example

ip nat inside source static 192.168.22.110 195.181.54.1
ip nat inside source static 192.168.22.111 195.181.54.2
!
interface fastethernet 0/0
 description Connected to ISP (Outside)
 ip address 83.90.42.1 255.255.255.252
 ip nat outside
!
interface fastethernet 0/1
 description Local private LAN (Inside)
 ip address 192.168.22.1 255.255.255.0
 ip nat inside

Dynamic NAT

Overloading

Overloading is often used, when you have a private internal LAN for example 192.168.1.0/24 and connect to the Internet through as ISP that lend you a Pulic IP address, through DHCP like a small SOHO router.

Overloading example 1: Connection with fixed WAN IP Address

Internal private network: 192.168.1.0/24
Fixed WAN IP address: 83.90.1.30/30

interface FastEthernet0/0
 description Inside. Internal LAN
 ip address 192.168.1.1 255.255.255.0
 ip nat inside
!
interface FastEthernet0/1
  description Outside: Internet connection to ISP
 ip address 83.90.1.30 255.255.255.252
 ip nat outside
!
ip nat inside source list 38 interface FastEthernet0/1 overload
!
access-list 38 permit 10.0.0.0 0.0.0.255

Overloading example 2: Connection with floating WAN IP Address (DHCP)

Internal private network: 192.168.1.0/24
WAN Address: DHCP

interface FastEthernet0/0
 description Inside. Internal LAN
 ip address 192.168.1.1 255.255.255.0
 ip nat inside
!
interface FastEthernet0/1
  description Outside: Internet connection to ISP
 ip address dhcp
 ip nat outside
!
ip nat inside source list 38 interface FastEthernet0/1 overload
!
access-list 38 permit 10.0.0.0 0.0.0.255

Overloading example 3: Cisco Router as a SOHO Router

Internal private network: 192.168.1.0/24 connected to fastethernet 0/0
WAN Address: DHCP client connected to fastethernet 0/1

Principle of SOHO Router

NAT, PAT (overloading) and DHCP configuration

ip dhcp excluded-address 192.168.1.1 192.168.100.99
ip dhcp excluded-address 192.168.1.200 192.168.100.255
!
ip dhcp pool INTERNAL-NET
   network 192.168.1.0 255.255.255.0
   domain-name jenshansen.dk
   default-router 192.168.1.1
   import all
!
interface FastEthernet0/0
 description Inside. Internal LAN
 ip address 192.168.1.1 255.255.255.0
 ip nat inside
!
interface FastEthernet0/1
 description Outside: Internet connection to ISP
 ip address dhcp
 ip nat outside
!
ip nat inside source list 1 interface FastEthernet0/1 overload
!
access-list 1 permit 192.168.1.0 0.0.0.255

The configration with an example of a fire-wall. Remember this config is at own risc!!

ip dhcp excluded-address 192.168.1.1 192.168.100.99
ip dhcp excluded-address 192.168.1.200 192.168.100.255
!
ip dhcp pool INTERNAL-NET
   network 192.168.1.0 255.255.255.0
   domain-name jenshansen.dk
   default-router 192.168.1.1
   import all
!
interface FastEthernet0/0
 description Inside. Internal LAN
 ip address 192.168.1.1 255.255.255.0
 ip nat inside
!
interface FastEthernet0/1
 description Outside: Internet connection to ISP
 ip address dhcp
 ip nat outside
 no ip source-route
!
ip nat inside source list 1 interface FastEthernet0/1 overload
!
access-list 1 permit 192.168.1.0 0.0.0.255
!
! Access-list 110. Incoming from Internet!
!  Add anti-spoofing entries.
access-list 110 deny ip 127.0.0.0 0.255.255.255 any
access-list 110 deny ip 192.0.2.0 0.0.0.255 any
access-list 110 deny ip 224.0.0.0 31.255.255.255 any
access-list 110 deny ip host 0.0.0.0 any
! Reject RFC1918 addresses from outside (Private)
access-list 110 deny ip 10.0.0.0 0.255.255.255 any
access-list 110 deny ip 172.16.0.0 0.15.255.255 any
access-list 110 deny ip 192.168.0.0 0.0.255.255 any
access-list 110 deny ip 169.254.0.0 0.0.255.255 any
! Permit ICMP return traffic but reject incoming
access-list 110 permit icmp any any echo-reply
access-list 110 permit icmp any any unreachable
access-list 110 permit icmp any any time-exceeded
access-list 110 deny   icmp any any
! Permit established connections
access-list 110 permit tcp any any established

Overlapping

Server load distribution - Load balancing between servers

You can load use nat pools to load balance between multiple IP addresses. Just remember a nat pool must be contiguous IP addresses.

Load balancing example

Load balancing between six WEB-servers to distribute the load among them.
In the real World www.tekkom.dk would for example resolve to IP Address 195.181.54.1 to which all users would connect.
In the figure below you see the company has six equal WEB-servers with the same content.

Load balance heavy user access to WEB-site between six WEB-servers

To load balance between several servers, you need to define a virtual IP address to which the users connect.

access-list 37 permit 195.181.54.1

See the full configuration below

ip nat pool WEB-SERVERS 192.168.22.9 192.168.22.14 netmask 255.255.255.0 type rotary
ip nat inside destination list 37 pool WEB-SERVERS
!
interface fastethernet 0/0
 description Connected to ISP (Outside)
 ip address 83.90.42.1 255.255.255.252
 ip nat outside
!
interface fastethernet 0/1
 description Local private LAN (Inside)
 ip address 192.168.22.1 255.255.255.0
 ip nat inside
!
access-list 37 permit 195.181.54.1

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