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Zorluk: ZorTroubleshooting Routing, Default Gateways, and ACLs

A network engineer is troubleshooting reachability issues for a newly provisioned monitoring server located in an enterprise branch office. The server is assigned the static IP address 10.64.36.19510.64.36.195 with a subnet mask of 255.255.255.192255.255.255.192 (/26/26). The default gateway configured on the server interface is 10.64.36.12910.64.36.129. The server can successfully ping its own IP address, but all attempts to reach the default gateway or remote networks fail with a destination host unreachable error. Which of the following identifies the root cause of this connectivity failure?

  1. The host IP address and the default gateway reside on different IP subnets due to an incorrect subnet mask configuration.Cevap
  2. B
    The IP address 10.64.36.195 is a reserved broadcast address for the local subnet and cannot process unicast ICMP traffic.
  3. C
    An implicit deny rule at the end of the router access control list is blocking ICMP echo reply traffic from the gateway interface.
  4. D
    The switchport connected to the monitoring server is configured on an unrouted native VLAN without an active IEEE 802.1Q trunk.

Cevap

The host IP address and the default gateway reside on different IP subnets due to an incorrect subnet mask configuration.
Under a 255.255.255.192 (/26) subnet mask, subnets increment by 64 in the fourth octet. The subnet containing host address 10.64.36.195 spans 10.64.36.192 to 10.64.36.255 (usable host range 10.64.36.193–10.64.36.254). The configured default gateway IP 10.64.36.129 falls into the previous subnet block spanning 10.64.36.128 to 10.64.36.191 (usable host range 10.64.36.129–10.64.36.190). Because a device's default gateway must reside on the same IP subnet as the host itself, local gateway reachability fails.

Adım Adım Çözüm

1
Analyze the subnet boundaries for the given host configuration.
A subnet mask of 255.255.255.192255.255.255.192 corresponds to a /26/26 prefix, dividing an octet into blocks of 6464 addresses (256−192=64256 - 192 = 64).
Determining block size is essential to find the network ID and usable range.
2
Determine the subnet block containing host IP 10.64.36.19510.64.36.195.
The block boundaries in the 4th octet are 0,64,128,1920, 64, 128, 192. Therefore, 10.64.36.19510.64.36.195 belongs to the subnet 10.64.36.192/2610.64.36.192/26. The usable host range for this subnet is 10.64.36.19310.64.36.193 to 10.64.36.25410.64.36.254, with broadcast address 10.64.36.25510.64.36.255.
Validates the network placement of the host.
3
Determine the subnet block containing gateway IP 10.64.36.12910.64.36.129.
The gateway address 10.64.36.12910.64.36.129 falls into the preceding block 10.64.36.128/2610.64.36.128/26 (usable range 10.64.36.12910.64.36.129 to 10.64.36.19010.64.36.190).
Validates the network placement of the default gateway.
4
Evaluate local subnet reachability.
Because the host and default gateway reside on different subnets under the /26/26 mask, the host cannot directly communicate with its default gateway without a routing boundary, causing unreachability to outside networks.
A host default gateway must reside on the same local Layer 3 broadcast domain as the host interface.

Anahtar Kavram

Default Gateway Subnet Boundary and Subnet Mask Alignment
Tahmini Süre:2m 0s
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Troubleshooting Routing, Default Gateways, and ACLs

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