Question

1. Using the handout given in class, describe NAT, how it works, and give an example of it using port numbers and IP addresses. Why is NAT useful? How many connections can a NAT support?
2. What is ICMP?
3. Draw four or five AS’s, and then label these with the routing algorithm that you think that your scenario is using. Then, draw the between AS routing algorithms. Describe the following possible networking algorithms and locate these on your hypothetical network drawing: RIP, OSPF, BGP, dynamic and static routing.
4. What are some “costs” associated with determining routing algorithms? What does the DV route based on? The OPSF?

Answer 1

Network Address Translation (NAT) is designed for IP address conservation. It enables private IP networks that use unregistered IP addresses to connect to the Internet. NAT operates on a router, usually connecting two networks together, and translates the private (not globally unique) addresses in the internal network into legal addresses, before packets are forwarded to another network.

As part of this capability, NAT can be configured to advertise only one address for the entire network to the outside world. This provides additional security by effectively hiding the entire internal network behind that address. NAT offers the dual functions of security and address conservation and is typically implemented in remote-access environments.

Basically, NAT allows a single device, such as a router, to act as an agent between the Internet (or public network) and a local network (or private network), which means that only a single unique IP address is required to represent an entire group of computers to anything outside their network.The order in which the transactions are processed using NAT is based on whether a packet is going from the inside network to the outside network or from the outside network to the inside network. Inside to outside translation occurs after routing, and outside to inside translation occurs before routing. Refer to NAT Order of Operation for more information.

NAT IP pools are a range of IP addresses that are allocated for NAT translation as needed. To define a pool, the configuration command is used:

ip nat pool <name> <start-ip> <end-ip> 
     {netmask <netmask> | prefix-length <prefix-length>}
     [type {rotary}]

Example 1

The following example translates between inside hosts addressed from either the 192.168.1.0 or 192.168.2.0 network to the globally unique 10.69.233.208/28 network:

ip nat pool net-208 10.69.233.208 10.69.233.223 prefix-length 28 
ip nat inside source list 1 pool net-208
!
interface ethernet 0
ip address 10.69.232.182 255.255.255.240
ip nat outside
!
interface ethernet 1
ip address 192.168.1.94 255.255.255.0
ip nat inside
!
access-list 1 permit 192.168.1.0 0.0.0.255
access-list 1 permit 192.168.2.0 0.0.0.255