The checksum of an IP packet is calculated over the entire header. Therefore, if the source or destination IP address or both change, the checksum must be recalculated. The same is true of the checksum in the TCP header. This number is calculated over the TCP header and data, and also over a pseudo-header that includes the source and destination IP addresses. Therefore, if an IP address or a port number changes, the TCP checksum must also change. Cisco’s NAT performs these checksum recalculations.