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Which command will allow a network administrator to check the IP address that is assigned to a particular MAC address?

• Router# show ip dhcp binding
• Router# show ip dhcp pool
• Router# show ip dhcp server statistics
• Router# show running-config I section_dhcp

The correct command to check the IP address that is assigned to a particular MAC address is:

Router# show ip dhcp binding

Explanation

When managing a network with DHCP (Dynamic Host Configuration Protocol), it’s essential for network administrators to be able to identify which IP addresses have been assigned to which devices, often based on their MAC (Media Access Control) addresses. This is particularly important for network troubleshooting, auditing, or managing IP address allocations.

Among the options provided, the show ip dhcp binding command is the one that directly displays a list of IP addresses assigned to clients by the DHCP server along with their associated MAC addresses. Here’s a deeper dive into what this command does and why it’s the most appropriate tool for this task.

What is DHCP?

DHCP is a network management protocol used to dynamically assign IP addresses to devices (known as DHCP clients) on a network. The protocol eliminates the need for manual IP address configuration, allowing devices to request an IP address from the DHCP server as soon as they join the network.

DHCP works in the following process:

1. DHCP Discover: A client sends out a broadcast request (DHCPDISCOVER) to find available DHCP servers.
2. DHCP Offer: The DHCP server responds with a DHCPOFFER, which includes an available IP address and other network configuration parameters like the subnet mask, default gateway, and DNS servers.
3. DHCP Request: The client sends a DHCPREQUEST message to request the offered IP address.
4. DHCP Acknowledgment: The server acknowledges the lease of the IP address with a DHCPACK message, allowing the client to use that address for a specified lease time.

Purpose of the show ip dhcp binding Command

The show ip dhcp binding command displays a table of IP addresses that have been assigned to clients by the DHCP server. This table is useful for monitoring which IP addresses are currently in use, which MAC addresses are using those IP addresses, and the lease duration for each IP address.

Output of the Command:

The typical output of the show ip dhcp binding command contains the following columns:

• IP Address: The dynamically assigned IP address to a device.
• Hardware Address: The MAC address of the device that has been assigned the IP address.
• Lease Expiration: The time remaining on the lease of the IP address.
• Type: Indicates whether the IP address assignment is automatic (from a pool) or manual.

An example of the output could look something like this:

Router# show ip dhcp binding
IP Address      Client-ID/Hardware Address    Lease Expiration        Type
192.168.1.100   0100.50ba.8e48.0a             May 20 2024 03:52 PM    Automatic
192.168.1.101   0100.12fa.4d67.b1             May 21 2024 10:23 AM    Automatic

In this table:

• IP Address: This column lists the IP addresses that the DHCP server has assigned.
• Client-ID/Hardware Address: This is the MAC address of the device to which the DHCP server assigned the IP address. The MAC address is an essential identifier, as it is globally unique to each network interface card (NIC).
• Lease Expiration: This column shows how long the device can use the assigned IP address before needing to renew the lease. The lease expiration time can help determine when IP addresses might become available again.
• Type: This refers to whether the IP address is assigned dynamically from a pool (automatic) or if it has been manually assigned.

When to Use the show ip dhcp binding Command

The show ip dhcp binding command is highly useful in various scenarios, such as:

1. Troubleshooting Network Issues: When devices on the network experience connectivity issues, it’s essential to know whether they are receiving the correct IP address. This command allows network administrators to verify if a device with a specific MAC address has been assigned the correct IP.
2. Monitoring IP Address Usage: By using this command, network administrators can keep track of which IP addresses are in use and by which devices. This is crucial for ensuring that IP addresses are not being misallocated or consumed unnecessarily.
3. Auditing and Security: In situations where security is a concern, this command helps track which devices (identified by their MAC addresses) are using which IP addresses. This information can be used to detect unauthorized devices on the network.
4. Managing DHCP Leases: If a device is experiencing issues due to an IP address conflict or a device is expected to release an IP soon, this command can be used to verify the status of leases and when they expire.

Comparison with Other Commands

Let’s compare the show ip dhcp binding command with the other commands mentioned, to highlight why this one is the most appropriate for checking the IP address associated with a specific MAC address.

1. show ip dhcp pool: This command displays information about the DHCP pools configured on the router, including how many IP addresses are available and how many have been allocated. While useful for monitoring pool usage, this command doesn’t provide the details necessary to check the specific IP-to-MAC mapping.

   Example Output:

   Router# show ip dhcp pool
   Pool MyPool
      Network: 192.168.1.0 /24
      Active Leases: 50
      Available: 200
   

   While this command tells you how many addresses are in use, it does not list which devices (MAC addresses) are using those IP addresses.

2. show ip dhcp server statistics: This command provides statistical information about the DHCP server’s performance, such as the number of DHCP Discover, Offer, Request, and Acknowledgment messages. Although it helps in monitoring server health and traffic, it does not show the mapping of IP addresses to MAC addresses.

   Example Output:

   Router# show ip dhcp server statistics
   Memory Usage       : 1356232
   IP Address Conflicts: 0
   DHCP Discover      : 1000
   DHCP Offer         : 900
   

3. show running-config I section_dhcp: This command extracts the DHCP-related configuration from the running configuration of the router. While it is useful for reviewing how DHCP is configured (e.g., pool settings, lease times), it does not display the actual binding of IP addresses to MAC addresses.

   Example Output:

   Router# show running-config I section_dhcp
   ip dhcp pool MyPool
      network 192.168.1.0 /24
      default-router 192.168.1.1
      lease 1 0 0
   

   This command helps in confirming the configuration but doesn’t provide the real-time IP-to-MAC binding.

Conclusion

The show ip dhcp binding command is the best choice when a network administrator wants to check the IP address assigned to a specific MAC address. This command provides a real-time listing of all devices (by MAC address) that have received an IP address from the DHCP server, along with information on their IP leases and expiration times. This detailed insight is crucial for managing, auditing, and troubleshooting a network that relies on DHCP for dynamic IP address assignment.

The other commands listed, while useful for specific tasks like monitoring pool usage or DHCP server performance, do not offer the same level of detail for individual device IP address assignments as show ip dhcp binding does.