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The OSI Network Layer

By Stephen Bucaro

If you have a very large network, you may divide the network into several segments and assign a network address to each segment. The Network layer, layer three of the OSI model, adds a source node address and a destination node address to each data packet. The Network layer uses logical addressing. Routable protocols at the Network layer deliver data packets to networks connected through routers.

Whereas the Data Link Layer (Layer 2) of the OSI model works with "frames", the Network Layer works with "packets". Routing protocols reside at the Network layer. Everything about routing is handled at the Network layer. The Network layer protocol adds a source node address and a destination node address to each data packet. Each LAN or network segment has a unique logical address.

Internet Control Message Protocol (ICMP)

ICMP is a Network layer protocol that reports on the success or failure of data delivery. It detects error conditions such as "Network unreachable" or "Access denied". It can indicate when part of a network is congested, when data fails to reach its destination, or when data has been discarded because the allotted time for its delivery (TTL) has expired.

ICMP anounces transmisssion failures to the sender, but ICMP cannot correct any of the errors it detects. The ICMP announcements provide information to upper-layer protocols so that they can route packets around problem areas.

Router

Routing is the process of moving data packets from one network or network segment to another. A packet that has a destination not on the local subnet is sent to a node called a default gateway that can communicate outside the local subnet.

Default Gateway Router

A router uses a routing table to determine which router is conected to the network that has the computer with the destination address. Routing protocols such as OSPF (Open Shortest Path First) Network layer are used to update the routing table.

OSI Router