IP Routing

1. Definition

• IP routing is the process of sending packets from a host on one network to another host on a different remote network through a router.
• Routers keep information about networks in a database called the routing table.
• Routers examine the destination IP address of a packet, determine the next-hop address, and forward the packet.
• Routers use routing tables to determine the next hop address to which the packet should be forwarded.

✅ Improved Explanation

• A router is used to connect different networks.
• Two devices are in different networks when:
  • They have different network IDs
  • OR different subnet masks
  • OR different IP ranges

Example

192.168.1.10 /24
192.168.2.10 /24

These are different networks → Router required

192.168.1.10 /24
192.168.1.20 /24

These are same network → Switch is enough

Routing Table

• Routing table is the table where all network information connected to a router is stored.
• It works like a database of routes for the router.
• It contains:
  • Destination network
  • Subnet mask
  • Next hop
  • Interface
  • Metric (cost)

Commands

To see routing table in router:

show ip route

To see MAC address table in switch:

show mac address-table

✅ Correction: You wrote sh mac-address-table → correct Cisco syntax is:

show mac address-table

Types of IP Routing

There are three types of IP routing:

1. Static Routing
2. Dynamic Routing
3. Default Routing

1. Static Routing

• In static routing, the administrator manually adds routes to the routing table.
• The administrator must configure each router manually.
• If any network change occurs, the administrator must update all routers.

✅ Improved: Static routing does NOT update automatically

Advantages of Static Routing

• No CPU overhead on router
• More secure (only defined routes allowed)
• No bandwidth usage between routers
• Predictable routing path

Disadvantages of Static Routing

• Requires full network knowledge
• Manual configuration on every router
• Not scalable
• Not suitable for large networks
• No automatic failover

Static Routing Configuration

Example:

Router(config)# ip route 192.168.2.0 255.255.255.0 10.0.0.2

Format:

ip route [destination network] [subnet mask] [next hop]

2. Dynamic Routing

Dynamic routing automatically learns routes using routing protocols.

Routers exchange routing information automatically.

Examples of Dynamic Routing Protocols

• RIP
• OSPF
• EIGRP
• BGP

Advantages

• Automatic updates
• Scalable
• Best path selection
• Automatic failover

Disadvantages

• Uses CPU resources
• Uses bandwidth
• More complex configuration

3. Default Routing

Default routing is used when router does not know the destination network.

Router sends packet to default route (gateway of last resort).

Example:

ip route 0.0.0.0 0.0.0.0 10.0.0.1

This means:

• Send all unknown traffic to 10.0.0.1

Used in:

• Small networks
• Internet access
• Stub networks

Quick Summary

Important Exam Notes

• Router connects different networks
• Switch connects same network
• Routing table stores network paths
• Static routing = manual
• Dynamic routing = automatic
• Default route = unknown destination route