The Rewrite

Source Port is Rewritten

The UDP source port changes with the return address, while the destination port stays fixed.

highlighted = computed this step

NAT changes the source port

The UDP source port changes from 40000 to 62000.

40000→6200040000\to62000
Source port rewriteThe before and after UDP headers show the source port rewrite.Before UDP header64 bits / 8 bytes0x9c400035000a0ee0byte 0byte 1byte 2byte 310011100010000000000000000110101byte 4byte 5byte 6byte 700000000000010100000111011100000source-port40000destination-port53length10checksum0x0ee0After UDP header64 bits / 8 bytes0xf2300035000a3e9cbyte 0byte 1byte 2byte 311110010001100000000000000110101byte 4byte 5byte 6byte 700000000000010100011111010011100source-port62000destination-port53length10checksum0x3e9c

Destination port stays put

The destination port remains 53.

destination port=53\text{destination port}=53
Source port rewriteThe before and after UDP headers show the source port rewrite.Before UDP header64 bits / 8 bytes0x9c400035000a0ee0byte 0byte 1byte 2byte 310011100010000000000000000110101byte 4byte 5byte 6byte 700000000000010100000111011100000source-port40000destination-port53length10checksum0x0ee0After UDP header64 bits / 8 bytes0xf2300035000a3e9cbyte 0byte 1byte 2byte 311110010001100000000000000110101byte 4byte 5byte 6byte 700000000000010100011111010011100source-port62000destination-port53length10checksum0x3e9c

Length stays put

The UDP length is still 10, because the payload size did not change.

UDP length=10\text{UDP length}=10
Source port rewriteThe before and after UDP headers show the source port rewrite.Before UDP header64 bits / 8 bytes0x9c400035000a0ee0byte 0byte 1byte 2byte 310011100010000000000000000110101byte 4byte 5byte 6byte 700000000000010100000111011100000source-port40000destination-port53length10checksum0x0ee0After UDP header64 bits / 8 bytes0xf2300035000a3e9cbyte 0byte 1byte 2byte 311110010001100000000000000110101byte 4byte 5byte 6byte 700000000000010100011111010011100source-port62000destination-port53length10checksum0x3e9c

Summary

This toy NAT step rewrites the source port and keeps the destination port.

source port after=62000\text{source port after}=62000
Source port rewriteThe before and after UDP headers show the source port rewrite.Before UDP header64 bits / 8 bytes0x9c400035000a0ee0byte 0byte 1byte 2byte 310011100010000000000000000110101byte 4byte 5byte 6byte 700000000000010100000111011100000source-port40000destination-port53length10checksum0x0ee0After UDP header64 bits / 8 bytes0xf2300035000a3e9cbyte 0byte 1byte 2byte 311110010001100000000000000110101byte 4byte 5byte 6byte 700000000000010100011111010011100source-port62000destination-port53length10checksum0x3e9c