Interpretation of Components of a Routing Table

A routing table is made up of entries that point to networks that are either directly connected, statically configured, or dynamically learned. The “show ip route” command can be used to inspect a routing table. Don’t forget that a routing table contains entries to networks that are either statically specified by the administrator or dynamically discovered by the routing protocols. The routing table contains roughly 24 codes.

Components of Routing Table:

A routing table comprises the necessary information in order to forward a particular packet via the best path suggestive of its destination. Here, each of the packets comprises information regarding its origin and destination. It is the job of the IP routing table to deliver the device with instructions in order to dispatch the packet through the next hop on its prescribed route across the network.

Prefix for the network: 

The number of IP addresses inside a specific host section of IP addresses is determined by the network prefix. A network prefix is an IP address grouping. IP addresses are represented as a prefix in CIDR notation, and a suffix is added to indicate how many bits are in the complete address. A slash mark (/) separates the suffix from the prefix. For the CIDR notation 192.2.1.9/24, for example, the prefix is 192.2.1.9 and the total number of bits in the address is 24. 128.208.0.0/24 tells me that the first 24 bits of the IP address are the “Network” part, and it also says the subnet mask, which in this case is 255.255.255.0. The prefix refers to the network prefix. In the routing table, there can be prefixes listed as /8, /24, /28. The prefix refers to the number of binary 1’s in the binary network mask: /8   = 11111111.00000000.00000000.00000000

Network Mask: 

An IP address is divided into subnets using a Netmask, a 32-bit “mask,” which also identifies the network’s available hosts. The numbers 0 and 255 are permanently assigned in a netmask and cannot be used. In this mask, 255.255.225.0, for instance, the “0” represents the allocated network address, whereas the “255” in this mask, 255.255.255.255, represents the assigned broadcast address. 255.255.255.0 255.255.255.255. A network’s size is determined by its network mask. However, in Dotted Decimal Notation, the network mask is the prefix:

• /8 = 11111111.00000000.00000000.00000000
• /8 = 255.0.0.0
• /28 = 11111111.11111111.11111111.11110000
• /28 = 255.255.255.240

A network mask specifies which bits of an IP address are network bits and which are host bits.

Next Hop:

The next hop is a routing word that refers to the next router that a packet can pass through. The next hop is the next probable destination for a data packet among the series of routers connected to a network. The phrase via is followed by the following hop ip address, thus when you read this in the table, it means 172.16.1.0 [120/1] via 10.10.10.3 00:00:19. 0/0 FastEthernet.  The address to which the router will pass packets is known as the next hop. As an illustration, the jumps from R1 to R4 are:

• R1 → R3 → R4
• R3 is the following hop for R4 after R1.

Protocol for routing: 

The output starts with a list of the routing protocol codes:

• A = Connected
• O = OSPF
• EIGRP = D
• R = RIP
• B = BGP

This code tells us where the routes came from before they were loaded into the FIB.

Administrative Distance :

Cisco routers compare routes with the same prefix and choose one based on administrative distance. Cisco prefers some routing protocols and route types over others. Administrative Distance and Metrics are held in square brackets. A list of administrative distances is provided below:

0     => Connected
1     => Static
20    => BGP (external routes)
90    => EIGRP (internal routes)
100   => IGRP
110   => OSPF
115   => IS-IS
120   => RIP
170   => EIGRP (external routes)
200   => BGP (internal routes)
254   => DHCP default route
255   => Unusable

Metrics:

Metric is the intrinsic measure of the route used by routing protocols. Various routing protocols use different metrics for this. However, in each scenario, a lower Metric denotes what the routing protocol views as a superior route.

Gateway to Last Resort:

The “gateway of last resort” is comparable to the “Default Gateway” or “Default Router,” but differs in that it will only be utilized if there is no other item in the routing table. In this scenario, every packet that was intended for a subnet that wasn’t in the routing table would be dropped.