What is Routing?
Routing refers to the process of relaying data between two or more different networks. It has become a critical component of the internet, which is constructed by interconnecting a vast number of network segments, particularly since the adoption of the IP protocol for communication. In the OSI reference model, routing operates at Layer 3 (Network Layer) and plays a key role in determining the communication paths for IP packets. Devices that perform routing are called routers.
To determine the optimal path for data transfer, a router references its routing table, which contains a list of destination routes. When the destination address of an incoming packet matches an entry in the routing table, the router forwards the packet to the appropriate route. This process is carried out by multiple routers in a "relay-like" fashion, ensuring that packets reach hosts on different network segments.
If a packet's destination address does not match any routes in the routing table, the router discards the packet and sends a destination unreachable notification to the sender via an ICMP packet. If multiple matching routes exist, the router selects the most optimal one for forwarding the packet.
Destination routes are added to the routing table through two methods:
The internet itself is a collection of multiple Autonomous Systems (AS). Routing between ASes is managed using an Exterior Gateway Protocol (EGP), while routing within an AS is handled using an Interior Gateway Protocol (IGP). Currently, BGP-4 (Border Gateway Protocol version 4) is widely used as an EGP, and OSPF (Open Shortest Path First) is commonly used as an IGP.