Tracert
Tracert (also called Traceroute) enables retrieval of the IP addresses of Layer 3 devices in the path to a destination. In the event of network failure, use tracert to test network connectivity and identify failed nodes.
Figure 2: Tracert operation
Tracert uses received ICMP error messages to get the IP addresses of devices. Tracert works as shown in Figure 2:
The source device sends a UDP packet with a TTL value of 1 to the destination device. The destination UDP port is not used by any application on the destination device.
The first hop (Device B, the first Layer 3 device that receives the packet) responds by sending a TTL-expired ICMP error message to the source, with its IP address (1.1.1.2) encapsulated. This way, the source device can get the address of the first Layer 3 device (1.1.1.2).
The source device sends a packet with a TTL value of 2 to the destination device.
The second hop (Device C) responds with a TTL-expired ICMP error message, which gives the source device the address of the second Layer 3 device (1.1.2.2).
This process continues until a packet sent by the source device reaches the ultimate destination device. Because no application uses the destination port specified in the packet, the destination device responds with a port-unreachable ICMP message to the source device, with its IP address encapsulated. This way, the source device gets the IP address of the destination device (1.1.3.2).
The source device determines that:
The packet has reached the destination device after receiving the port-unreachable ICMP message.
The path to the destination device is 1.1.1.2 to 1.1.2.2 to 1.1.3.2.