laag3

laag3, also known as Layer 3, is the third layer of the OSI model and corresponds to the Network layer in the TCP/IP model. It is responsible for moving packets between networks by selecting routes and providing logical addressing.

Core responsibilities include logical addressing using IP addresses, routing decisions based on routing tables and algorithms, and the actual forwarding of packets by routers and, in some cases, Layer 3 switches. Layer 3 devices determine the next-hop toward the destination and handle inter-network communication.

Layer 3 devices exchange reachability information using routing protocols such as static routes, RIP, OSPF, IS-IS, and BGP. IP is the primary protocol at this layer, with both IPv4 and IPv6 variants in common use. Key concepts include subnetting and CIDR, which define how address spaces are partitioned and routed. Packet fragmentation can occur at this layer when a packet crosses networks with smaller MTUs.

Security and policy at laag3 are enforced with access control lists and routing filters, while quality of service can be applied to manage flow between subnets. Layer 3 also supports features such as route aggregation and policy-based routing to influence path selection.

In practical networks, laag3 is implemented by routers and often by Layer 3 switches that perform routing at high speed, enabling inter-VLAN routing and scalable inter-network connectivity. This layer thus provides end-to-end connectivity across multiple networks while relying on Layer 2 for local frame forwarding.

Relation to other layers: laag3 relies on Layer 2 to forward frames within a local network, while providing inter-network connectivity across multiple networks. In the TCP/IP model, laag3 maps to the Internet/Network layer.