A network security analyst auditing firewall logs observes traffic originating from an int… — CompTIA Network+

A network security analyst auditing firewall logs observes traffic originating from an internal host with IP address $10.50.84.142/21$ . To configure a targeted network access rule, the analyst must determine the exact boundaries of the local subnet. Which of the following identifies the correct network address and broadcast address for the subnet containing this host?

Network address $10.50.80.0$ and broadcast address $10.50.87.255$ Answer
B
Network address $10.50.84.0$ and broadcast address $10.50.84.255$
C
Network address $10.50.80.0$ and broadcast address $10.50.88.255$
D
Network address $10.50.80.1$ and broadcast address $10.50.87.254$

Answer

Network address

10.50.80.0

and broadcast address

10.50.87.255

For the host IP address

10.50.84.142/21

, the

/21

mask corresponds to

255.255.248.0

. The third octet operates in block increments of

8

. The network boundary starts at

80

(

10 \times 8

), making the network address

10.50.80.0

. The subnet spans from

10.50.80.0

through

10.50.87.255

, establishing

10.50.87.255

as the broadcast address.

Step-by-Step Solution

Determine the subnet mask from the CIDR prefix length

/21

/21

mask has

21

network bits:

255.255.248.0

Converting CIDR notation to dotted-decimal format identifies which octet contains the subnet boundary.

Calculate the block size (increment) in the third octet.

Block size =

256 - 248 = 8

Subnet boundaries in the third octet recur in multiples of

8

Identify the network address by finding the highest multiple of

8

less than or equal to the host's third octet (

84

Multiples of

8

0, 8, 16, \dots, 72, 80, 88

. Since

80 \leq 84 < 88

, the network third octet is

80

. Network address =

10.50.80.0

The network address uses all zeros in the host portion.

Calculate the broadcast address for this subnet.

The next subnet begins at

10.50.88.0

, so the broadcast address is

10.50.87.255

The broadcast address is one IP less than the next subnet's network address (all host bits set to 1).

Key Concept

IPv4 Subnet Boundaries and Broadcast Address Calculation

Estimated Time:1m 30s

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