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Difficulty: MediumTroubleshooting IP Addressing and DHCP Services

A network administrator receives reports that mobile devices connecting to the guest Wi-Fi network (VLAN 85, subnet 172.20.85.0/24172.20.85.0/24) are failing to access internet resources. Inspecting a network details page on an affected device reveals an IP address of 169.254.41.19169.254.41.19 with a subnet mask of 255.255.0.0255.255.0.0. Reviewing the DHCP server statistics shows that the active pool for VLAN 85 currently has 00 available IP addresses remaining out of the 254254 allocated pool entries. Which of the following actions should the administrator take to resolve the root cause of this issue?

  1. Expand the DHCP scope for VLAN 85 by modifying the subnet mask to a /23/23 or shortening the DHCP lease duration.Answer
  2. B
    Manually configure a static default gateway on the affected client devices to override the automatic private IP assignment.
  3. C
    Configure an IP helper address on the wireless access point to forward broadcast requests across VLAN boundaries.
  4. D
    Increase the usable pool capacity by configuring the DHCP server to assign the network ID and broadcast address to clients.

Answer

Expanding the DHCP scope for VLAN 85 (such as by modifying the subnet mask to a /23) or reducing the lease duration will resolve the address pool exhaustion issue.
The client receiving an APIPA address (169.254.x.x169.254.x.x) combined with the DHCP server showing 00 available addresses indicates DHCP scope exhaustion. To remediate scope exhaustion in high-turnover guest wireless networks, administrators should expand the address pool (for example, expanding a /24/24 subnet to a /23/23) or decrease the DHCP lease duration so inactive leases expire sooner.

Step-by-Step Solution

1
Analyze the client symptom and IP address output.
The IP address 169.254.41.19169.254.41.19 is an Automatic Private IP Addressing (APIPA) address, indicating the client failed to obtain a DHCP lease.
When a client cannot reach a DHCP server or receive an available lease, it self-assigns an APIPA address in the 169.254.0.0/16169.254.0.0/16 range.
2
Examine the DHCP server pool statistics.
The server logs show 00 available addresses remaining out of 254254 total entries in the scope.
This confirms DHCP scope exhaustion, where all usable host addresses have been leased out to existing clients.
3
Identify the appropriate remediation strategy.
Shortening lease times allows inactive leases to expire faster, while expanding the subnet mask to /23/23 increases usable addresses from 254254 to 510510.
Increasing scope capacity or accelerating lease turnover directly resolves scope exhaustion in high-density or guest wireless environments.

Key Concept

DHCP Scope Exhaustion and APIPA Diagnosis
Estimated Time:1m 30s
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Topics

Troubleshooting IP Addressing and DHCP Services

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