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Zorluk: OrtaTroubleshooting Routing, Default Gateways, and ACLs

A network technician is configuring a newly deployed server in a corporate data center. The server is assigned the IPv4 address 10.100.15.7010.100.15.70 with a subnet mask of 255.255.255.192255.255.255.192 (/26) and a default gateway of 10.100.15.110.100.15.1. During testing, the server can communicate with local hosts in the 10.100.15.64/2610.100.15.64/26 IP range, but it cannot access devices on remote subnets or reach the default gateway. Which of the following best explains why the server cannot reach its default gateway?

  1. The server IP address and default gateway reside on different IP subnets due to the configured subnet mask length.Cevap
  2. B
    The default gateway address 10.100.15.1 is the subnet broadcast address and cannot be assigned to a router interface.
  3. C
    The server has auto-configured an APIPA address because it failed to obtain a DHCP reservation from the default gateway.
  4. D
    An implicit deny rule on the default gateway router is dropping all outbound ARP requests originating from the server.

Cevap

The server IP address and default gateway reside on different IP subnets due to the configured subnet mask length.
With a subnet mask of 255.255.255.192255.255.255.192 (/26), the network is partitioned into subnets of 64 addresses each. The host address 10.100.15.7010.100.15.70 belongs to the subnet span 10.100.15.6410.100.15.64 through 10.100.15.12710.100.15.127. However, the configured default gateway address 10.100.15.110.100.15.1 belongs to the previous subnet span 10.100.15.010.100.15.0 through 10.100.15.6310.100.15.63. Because a host and its default gateway must reside within the same local IPv4 subnet to establish Layer 2 ARP adjacency, the server cannot send traffic outside its local segment.

Adım Adım Çözüm

1
Determine the subnet boundaries for host IP 10.100.15.7010.100.15.70 with mask 255.255.255.192255.255.255.192 (/26).
A /26 mask has a block size of 256−192=64256 - 192 = 64. The subnets are 10.100.15.0/2610.100.15.0/26 (10.100.15.010.100.15.0 to 10.100.15.6310.100.15.63) and 10.100.15.64/2610.100.15.64/26 (10.100.15.6410.100.15.64 to 10.100.15.12710.100.15.127).
Calculating subnet ranges identifies which addresses belong to the same local broadcast domain.
2
Compare the placement of the server IP and default gateway IP within the calculated subnets.
The server IP (10.100.15.7010.100.15.70) falls into the 10.100.15.64/2610.100.15.64/26 subnet, whereas the default gateway IP (10.100.15.110.100.15.1) falls into the 10.100.15.0/2610.100.15.0/26 subnet.
A host can only communicate directly with its default gateway if both share the same IP subnet.
3
Formulate the root cause diagnosis.
Because the server and gateway reside on different subnets, the server considers the gateway off-link and cannot perform direct Layer 2 ARP resolution to reach it.
This subnet mismatch prevents off-subnet traffic routing.

Anahtar Kavram

Default Gateway Subnet Mismatch
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Konular

Troubleshooting Routing, Default Gateways, and ACLs

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