layer3

Layer 3, also known as the network layer in the OSI model, is responsible for moving packets between networks. It handles logical addressing, routing, and fragmentation. Devices at this layer, primarily routers and Layer 3 switches, examine a packet's destination IP address to determine an appropriate path toward the final host. Layer 3 provides end-to-end delivery across multiple networks by selecting routes and maintaining routing tables.

Key functions include addressing (IPv4 and IPv6), routing, and fragmentation/reassembly when packets must traverse networks with different MTUs. Forwarding decisions are made using routing protocols such as OSPF, BGP, RIP, and EIGRP, which exchange topology information to build and update routing tables. Routers may perform NAT or VPN termination at the network edge; Layer 3 devices can also perform inter-VLAN routing when configured as Layer 3 switches.

Protocols associated with Layer 3 primarily include IP (IPv4 and IPv6) and ICMP for error messages and diagnostics. Addressing standards define how hosts are identified and located, while ARP serves to resolve IP addresses to MAC addresses within a local link, sitting at the intersection of Layer 2 and Layer 3.

IPv6 introduces features such as neighbor discovery and simplified header format, while IPv4 remains dominant in many networks. Fragmentation can occur at the source or at a router along the path in IPv4; ICMP messages assist in path MTU discovery and error reporting.

Security and management considerations at Layer 3 include access control lists, firewalls, and routing protocol authentication. Layer 3 is distinct from Layer 2 in that it routes based on logical addresses rather than physical addresses, enabling inter-network communication and wider Internet connectivity.

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