How to know the ip address of the device connected to a physical interface

Last updated: Dec 18, 2024

In this article, we'll look at how to map a device's IP address to the physical interface of a Cisco switch.

We'll do this in two stages:
- First, we'll map the mac address to the physical port using the mac table.
- Then we'll use the arp table to map the mac address to the ip address.

Prerequisite: The switch must have an IP address configured in the same VLAN as the devices whose port we want to know. (I'll explain how to do this below).

Network Architecture

Let's assume that we're in the case of the architecture below, and that we want to know which IP addresses are connected to interfaces Gi1/0/1 and Gi1/0/2.

Match IP addresses to physical ports

Set an IP address for the switch

Note: The switch address(es) must be in the same vlan as the devices to be identified. In this example, all devices to be identified are in vlan 1.

Configure vlan 1 interface:

switch01(config)# interface vlan 1
switch01(config-if)# ip address 192.168.1.1 255.255.255.0
switch01(config-if)# no shutdown

Get mac addresses

As explained above, switches are Layer 2 devices, so there's no way to retrieve this information with a single command. We must therefore first retrieve the mac addresses of the devices and then search inside the arp table for the correspondence between the mac address and the IP address.

Let's see how to display the mac addresses connected to our two physical ports.

Display the mac address of the device connected to interface Gi1/0/1:

switch01# show mac address-table | include Gi1/0/1
   1    24b6.fd14.0853    DYNAMIC     Gi1/0/1

Display the mac address of the device connected to interface Gi1/0/2:

switch01# show mac address-table | include Gi1/0/2
   1    f8db.8845.ef15    DYNAMIC     Gi1/0/2

Note the informations retrieved:

Interface	mac address	ip address
Gi1/0/1	24b6.fd14.0853	-
Gi1/0/2	f8db.8845.ef15	-

Retrieve the IP addresses

Ping the IP addresses we wish to identify in ordre to populate the switch's arp table:

switch01# ping 192.168.1.200
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 192.168.1.200, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 36/51/74 ms

switch01# ping 192.168.1.210
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 192.168.1.210, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 33/48/71 ms

Display the arp table to obtain the correspondence between the IP address and the mac address (use the “include” instruction to display only the line corresponding to our mac address):

switch01# show arp | include 24b6.fd14.0853
Internet  192.168.1.210              0   24b6.fd14.0853  ARPA   Vlan1

switch01# show arp | include f8db.8845.ef15
Internet  192.168.1.200              0   f8db.8845.ef15  ARPA   Vlan1

We can deduce from this information that the device with IP address 192.168.1.210 is connected to interface Gi1/0/1 and that the device with IP address 192.168.1.200 is connected to interface Gi1/0/2.

Interface	mac address	ip address
Gi1/0/1	24b6.fd14.0853	192.168.1.210
Gi1/0/2	f8db.8845.ef15	192.168.1.200

Computing

Music

Misc