Interview questions on network address translation

Name one instance where static NAT is used in a real world deployment

It is used for mapping a public IP address for a Server with a private IP address.

Why does Active FTP not work with NAT in an Internet environment.

In Active FTP, the data connection is established to a port on the FTP client by the FTP server. The port number along with the IP address to which the server needs to initiate the connection is provided by the FTP client after the control connection is succesfull. When the client is behind the NAT router, the FTP server cannot initiate the connection to the provided IP address , as typically it would be a private IP address not routable on the internet.

How does NAT work in situations where transport layer protocols are not used. For ex: Ping

Ping does not use transport layer protocols. It uses ICMP at the network layer. NAT uses the sequence number field in the ICMP header to identify packets on which NAT is applied.

Two computers are behind a NAT router. The computers use the routers public IP address for sharing internet connection.If a user on the internet pings the public IP address of the router, which device would respond

The router would respond as it is configured for the public ip address.

How many times can NAT be applied on a packet before it reaches the destination

Any number of times.

Give a good reason as to why a NAT router is preferred over a Proxy for sharing internet connection

NAT works at the network layer. This means that irrespective of the application, all packets can be sent out on the internet. Proxy is application specific. So if a HTTP proxy is deployed, it can send out only HTTP based traffic on to the internet.Other traffic like ping, FTP etc would be blocked.

Does TCP checksum change after NAT is applied

TCP checksums are calculated based on a pseudo header which also includes source IP address of the IP header. Since, the source IP header address is modified when NAT is applied, the checksum would be affected.

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