1. CISCO NETWORKING ACADEMY Chabot College ELEC 99.08 Network Address Translation

2. CISCO NETWORKING ACADEMY Shortcomings of Subnetting: Waste (up to 50%) –Lots of addresses are unusable: first & last subnet first & last host in each subnet Rigidity –Subnet structure is inflexible same size for every subnet subnet with largest no. of hosts determines size for all

3. CISCO NETWORKING ACADEMY Case Study: 4CNet IP Address Allocation to Chabot-Las Positas Algorithm: #hosts/254 = #class Cs Fails to consider network structure. Subnets needed for: –Broacast control –WAN links –Access policy enforcement

4. CISCO NETWORKING ACADEMY Case Study: 4CNet IP Address Allocation to Chabot-Las Positas Can we afford the waste & rigidity of subnetting? YES, if we run NAT & a large private address space.

5. CISCO NETWORKING ACADEMY Network Address Translation (NAT) Private address space inside network 4CNet-assigned addresses outside network Translation performed by Cisco PIX (Private-Internet Exchange) (Can also be performed by a router.)

6. CISCO NETWORKING ACADEMY How NAT works...

7. CISCO NETWORKING ACADEMY Default Translation The PIX assigns a single address to all traffic sent to the internet. The PIX uses TCP sequence numbers to map the returning traffic from established sessions to internal host addresses. Supports > 16,000 simultaneous sessions.

8. CISCO NETWORKING ACADEMY Address Mapping Internal hosts can be mapped to both internal (private) and external IP addresses:

9. CISCO NETWORKING ACADEMY What problems does NAT solve? Allows a huge address space for your net. Allows creation of many subnets with many hosts. (e.g. Class B network subnetted 255.255.255.0). Address waste doesn’t matter. Rigidity doesn’t matter (subnets can be oversized to allow for growth) Allows flexible access policies and firewalling.

10. CISCO NETWORKING ACADEMY What problems does NAT solve? …and you’ll never have to renumber your network again.

11. CISCO NETWORKING ACADEMY What tradeoffs result? Capital cost of the box. Single point of failure in connection to internet. Dual DNS required… But this is also an advantage: lets you decide what the world gets to learn about your internal network.

12. CISCO NETWORKING ACADEMY Dual DNS