What is IP Routing? 83
What is IP Routing? An IP router, unlike a bridge, operates at the network layer of the OSI
Reference Model. The network layer is also referred to as Layer 3. An IP
router routes packets by examining the network layer address (IP
address). Bridges use data link layer MAC addresses (at Layer 2) to
perform forwarding. See Figure 22
.
Figure 22 OSI Reference Model and IP Routing
When an IP router sends a packet, it does not know the complete path to
a destination — only the next hop (the next device on the path to the
destination). Each hop involves three steps:
1 The IP routing algorithm computes the next hop IP address and the next
router interface, using routing table entries.
2 The Address Resolution Protocol (ARP) translates the next hop IP address
into a physical MAC address.
3 The router sends the packet over the network across the next hop.
Application layer
Presentation layer
Session layer
Transport layer
Network layer
Data link layer
Physical layer
MAC
IP
OSI Reference Model
ARP
RIP
ICMP
OSPF
IP Routing
Bridging
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