A network administrator is troubleshooting network reachability for a newly installed serv… — CompTIA Network+

A network administrator is troubleshooting network reachability for a newly installed server in an enterprise datacenter. The host is assigned the IPv4 address $172.16.50.130$ with a subnet mask of $255.255.255.192$ ( $/26$ ). The administrator discovers that while the host can ping its local loopback address, it cannot establish SSH connections to a remote server at $172.16.50.200/26$ or reach external networks.

The host IP configuration is verified as follows:
- IPv4 Address: $172.16.50.130$
- Subnet Mask: $255.255.255.192$
- Default Gateway: $172.16.50.126$

The router interface servicing this VLAN is configured as follows:
text
interface GigabitEthernet0/0.50
encapsulation dot1Q 50
ip address 172.16.50.129 255.255.255.192
ip access-group VLAN50_IN in
!
ip access-list extended VLAN50_IN
permit tcp 172.16.50.128 0.0.0.63 any eq 22
permit icmp 172.16.50.128 0.0.0.63 any

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

The configured default gateway ( $172.16.50.126$ ) resides in a different IP subnet than the host ( $172.16.50.128/26$ ), preventing ARP resolution for off-subnet destinations.Cevap
B
The access control list `VLAN50_IN` lacks an explicit permit statement for return TCP traffic, causing the implicit deny rule to drop incoming SSH responses.
C
The target server at $172.16.50.200$ is located in the same broadcast domain as the source host, making default gateway routing unnecessary.
D
The $/26$ subnet mask restricts the host capacity of Subnet 2 to 30 usable addresses, making $172.16.50.130$ an invalid host IP.

Cevap

The primary root cause of the connectivity failure is that the configured default gateway (172.16.50.126) resides on a different IP subnet than the source host (172.16.50.128/26), which prevents Layer 2 ARP resolution for off-subnet traffic.

The host IP address

172.16.50.130

with mask

255.255.255.192

(

/26

) places the host in the

172.16.50.128/26

subnet, which has a usable address range of

172.16.50.129

through

172.16.50.190

. The configured default gateway (

172.16.50.126

) falls into the preceding subnet (

172.16.50.64/26

). Because the gateway address is outside the host's local subnet, the host cannot send ARP requests to resolve the gateway's MAC address, completely blocking off-subnet routing.

Adım Adım Çözüm

Calculate the subnet boundaries for the host IP address using the subnet mask

255.255.255.192

(

/26

/26

mask yields block sizes of 64 (

256 - 192 = 64

). Subnet ranges are Subnet 0 (

172.16.50.0/26

), Subnet 1 (

172.16.50.64/26

), Subnet 2 (

172.16.50.128/26

), and Subnet 3 (

172.16.50.192/26

Determining exact subnet boundaries establishes which IP addresses reside on the same broadcast domain.

Identify the subnet belonging to the host IP

172.16.50.130

and determine its valid usable IP range.

The host IP

172.16.50.130

falls into Subnet 2 (

172.16.50.128/26

). Network address is

172.16.50.128

, broadcast address is

172.16.50.191

, and usable host IPs range from

172.16.50.129

172.16.50.190

Host IP verification confirms that

172.16.50.130

is a valid usable address within Subnet 2.

Evaluate the location of the configured default gateway IP

172.16.50.126

relative to Subnet 2.

The IP

172.16.50.126

falls into Subnet 1 (

172.16.50.64/26

), where usable IPs span

172.16.50.65

172.16.50.126

. The router's actual interface IP for VLAN 50 is

172.16.50.129

A host must have a default gateway that resides within its own local IP subnet to complete local ARP requests and forward frames to its Layer 3 exit point.

Determine the impact of a default gateway subnet mismatch on traffic forwarding.

When the host attempts to reach an off-subnet destination (

172.16.50.200

), it determines that

172.16.50.126

is off-link (outside

172.16.50.128/26

) and fails to generate ARP requests for

172.16.50.126

, dropping packet transmission locally.

Hosts cannot send traffic to a default gateway that does not share their local subnet.

Anahtar Kavram

Default Gateway Subnet Mismatch Diagnosis

Tahmini Süre:3m 0s

Soru

Cevap

Adım Adım Çözüm

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