TCP/IP Network Layer (Layer 3)

Layer 3 is the Network or Internet layer.

When transmitting data, this layer adds a header containing the source and destination IP addresses to the to the data received from the Transport layer. The packet it creates will then be forwarded to the MAC or Data Link layer.

When receiving data, this layer is used to determine if the packet received by the host contains the host’s IP address. If it does, the data is forwarded up to the Transport layer.

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Click image to enlarge.

Routers are referred to as “layer 3” devices because they route packets based on their IP addresses.

TCP/IP IPv4 Packet Header

The Network layer header includes more than just the source and destination IP addresses.

IPv4_header.JPG

For more information on IPv4: http://en.wikipedia.org/wiki/IPv4
For more information on IPv6: http://en.wikipedia.org/wiki/IPv6

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