Category: Cisco

Here are some NAT troubleshooting tips:

• show ip nat translations
• show running-config
• show ip nat statistatics
• debug ip nat
• clear ip nat translations *
• Check inside and outside are placed on the correct interfaces
• Ensure the natpool covers the correct range of IPs
• Check the name of the pools are matched – I made a typo during testing and as a result NAT didn’t work
• Ensure the access-list is configured correctly to allow translation

Showing examples of Dynamic, Static and Nat overload.

Configuring NAT Overload on Router1:

interface FastEthernet0/0
ip address 10.0.0.1 255.255.255.224
ip nat inside

interface Serial0/1/0
ip address 62.0.0.2 255.255.255.0
ip nat outside

ip nat pool natpool 62.0.0.2 62.0.0.2 netmask 255.255.255.0
ip nat inside source list 1 pool natpool overload

access-list 1 permit 10.0.0.0 0.0.0.31

Configuring Static NAT on Router1:

interface FastEthernet0/0
ip address 10.0.0.1 255.255.255.224
ip nat inside

interface Serial0/1/0
ip address 62.0.0.2 255.255.255.0
ip nat outside

ip nat inside source static 10.0.0.2 62.0.0.2
ip nat inside source static 10.0.0.3 62.0.0.4

Configuring Dynamic NAT on Router1:

interface FastEthernet0/0
ip address 10.0.0.1 255.255.255.224
ip nat inside

interface Serial0/1/0
ip address 62.0.0.2 255.255.255.0
ip nat outside

ip nat pool natpool 62.0.0.3 62.0.0.4 netmask 255.255.255.0
ip nat inside source list 1 pool natpool

access-list 1 permit 10.0.0.0 0.0.0.31

Network Address Translation allows private IPv4 addresses to be translated to an IPv4 public address to communicate over the Internet. This saves public IP addresses as not every device needs a public IP address and if we allowed that we would’ve run out of IPv4 addresses by now.

NAT allows multiple private IPv4 addresses to be translated to one public IP address, therefore saving public IP addresses.

There are three different types of NAT.

1. Static NAT – This is a one-to-one translation, one private IP address will be translated to one public IP address
2. Dynamic NAT – You don’t need to manually map each IP static with dynamic NAT but you do need to ensure you have enough public IPs for the private IPs to be translated to.
3. Overloading (PAT) – This is a many-to-one translation, multiple private IP addresses will be translated to one public IP, kinda cool right? This is the reason we haven’t run out of IPv4 addresses yet.

Some tips when troubleshooting ACL:

• Ensure correct IP and wildcast masks are correctly entered into the ACL
• Ensure an access-group is applied to an interface
• If no traffic is permitted, all traffic will be denied, there is an explicit deny.
• Access-lists are read top to bottom, if a first match is found it will stop reading. So if a deny is specified it may block a permit statement. Order of ACL is important.
• Remarks can be added to ACL to make reading them in future easier using the access-list <number> remark “This ACL blocks FTP”

The following commands can be used on Router to verify and monitor ACL’s :

• show access-list
• show access-list 101
• show ip access-list
• show ip interfaces
• show running-config

It is possible to verify connectivity depending on the ACL by using applications such as FTP, Telnet and SSH from an end device such as a PC/Laptop.