As you know by now, IP Addressing is an integral part of networking and given the complexity of addressing and subnetting, it is common to have IP addressing errors in the network. So it is essential for you to be able to troubleshoot common problems related to IP Addressing. Before troubleshooting a network, you have to understand the below given common protocols and utilities that are used to troubleshoot:
Packet InterNet Grouper (PING) – Ping is one of the most commonly used utility that is used to troubleshoot addressing and connectivity problems. This utility is available in almost all operating systems, including Cisco devices and can be accessed by the command line interface using the ping command. It uses the ICMP protocol to check if the destination host is live or not.
Traceroute – Traceroute is another common utility that is available with all operating systems. In some operating systems the utility can be access using the tracert or traceroute command on the CLI. It is used to find each hop between the source and destination hosts and is useful to see the path taken by a packet.
ARP table – Sometimes it is useful to look at the ARP table of a system. This table contains the MAC address to IP address bindings learned by the system. On most operating systems the ARP table can be viewed using the arp –a command. On a Cisco device the arp table can be viewed using the show ip arp command.
IP config – Sometimes, you need to verify the IP address, subnet mask, default gateway and DNS addresses the host is using. On a windows machine all this information can be seen in the output of the ipconfig /all command. On a unix based system, this information can be seen using the ifconfig command.
For the following section consider the network shown in Figure 2-6. In this network, HostA is trying to reach ServerA and ServerB but is not able to.
Before looking at the IP addressing, you should quickly check network connectivity using four steps that Cisco recommends:
1. Ping 127.0.0.1, the loopback address from the Host. You will need to open a terminal window of your operating system to use the ping utility. If you get an output similar to the following, it shows that the IP stack in the host is working well:
ping 127.0.0.1
PING 127.0.0.1 (127.0.0.1): 56 data bytes
64 bytes from 127.0.0.1: icmp_seq=0 ttl=64 time=0.073 ms
64 bytes from 127.0.0.1: icmp_seq=1 ttl=64 time=0.096 ms
64 bytes from 127.0.0.1: icmp_seq=2 ttl=64 time=0.095 ms
64 bytes from 127.0.0.1: icmp_seq=3 ttl=64 time=0.145 ms