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Zorluk: OrtaTroubleshooting Routing, Default Gateways, and ACLs

A network administrator configures a workstation with IP address 172.20.30.45172.20.30.45 and subnet mask 255.255.255.192255.255.255.192. The workstation can successfully ping local hosts, but fails to reach any external subnets. Reviewing `ipconfig /all` reveals the following output:

IPv4 Address. . . . . . . . . . . : 172.20.30.45
Subnet Mask . . . . . . . . . . . : 255.255.255.192
Default Gateway . . . . . . . . . : 172.20.30.65

Which of the following identifies the root cause of the connectivity failure?

  1. The default gateway IP address resides outside the workstation's local subnet range.Cevap
  2. B
    An implicit deny rule on the router's inbound Access Control List (ACL) is dropping outbound packets.
  3. C
    The host IP address 172.20.30.45 is a directed broadcast address and cannot be assigned to an end device.
  4. D
    The workstation requires switchport re-assignment because inter-VLAN routing is disabled on the switch.

Cevap

The default gateway IP address resides outside the workstation's local subnet range.
The workstation IP address (172.20.30.45172.20.30.45) with subnet mask 255.255.255.192255.255.255.192 (/26) belongs to the subnet 172.20.30.0/26172.20.30.0/26, which spans addresses 172.20.30.0172.20.30.0 through 172.20.30.63172.20.30.63. The configured default gateway (172.20.30.65172.20.30.65) lies within the adjacent subnet 172.20.30.64/26172.20.30.64/26. Because a host's default gateway must reside on the same local subnet as the host interface, the host cannot send local ARP queries for the gateway, breaking all outbound non-local traffic.

Adım Adım Çözüm

1
Calculate the subnet boundaries for the workstation using the given CIDR mask
A subnet mask of 255.255.255.192255.255.255.192 corresponds to a /26/26 prefix, giving a block size of 256−192=64256 - 192 = 64 addresses per subnet.
Subnet ranges increment by 64 in the fourth octet: Subnet 0 (172.20.30.0−172.20.30.63172.20.30.0 - 172.20.30.63) and Subnet 1 (172.20.30.64−172.20.30.127172.20.30.64 - 172.20.30.127).
2
Determine the host's subnet and valid host range
The host address 172.20.30.45172.20.30.45 falls into Subnet 0 (172.20.30.0/26172.20.30.0/26), where usable IPs are 172.20.30.1172.20.30.1 through 172.20.30.62172.20.30.62.
Devices must have a default gateway configured within their own local subnet boundary to enable ARP resolution for off-subnet forwarding.
3
Evaluate the configured default gateway address
The configured gateway IP is 172.20.30.65172.20.30.65, which falls into Subnet 1 (172.20.30.64/26172.20.30.64/26).
Because 172.20.30.65172.20.30.65 is on a different subnet, the host cannot ARP for its gateway address directly on the local link.

Anahtar Kavram

Default Gateway Subnet Alignment
Tahmini Süre:1m 30s
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Troubleshooting Routing, Default Gateways, and ACLs

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