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Difficulty: EasyIPv4 Addressing and Subnetting

A network administrator is assigning an IP address to a router interface using a CIDR prefix length of /26/26. Which dotted-decimal subnet mask corresponds to this prefix?

  1. A
    255.255.255.128
  2. 255.255.255.192Answer
  3. C
    255.255.255.224
  4. D
    255.255.255.240

Answer

255.255.255.192
A CIDR prefix of /26 indicates that 26 bits are set to 1 in the subnet mask. The first 24 bits populate the first three octets fully as 255.255.255. The remaining 2 network bits in the fourth octet correspond to binary 11000000, which equals 128+64=192128 + 64 = 192, producing the dotted-decimal subnet mask 255.255.255.192.

Step-by-Step Solution

1
Determine the bit allocation across octets for a /26 prefix.
The first three octets use 24 bits (8 + 8 + 8 = 24), leaving 2 network bits for the fourth octet.
An IPv4 subnet mask has 32 total bits.
2
Convert the 2 network bits in the fourth octet to decimal.
Binary 11000000 equals 128+64=192128 + 64 = 192.
Subnet mask bits are filled from left to right.
3
Combine all four octets in dotted-decimal format.
255.255.255.192
The first three octets consist of 8 set bits each, yielding 255.255.255.

Key Concept

Converting CIDR notation to dotted-decimal subnet masks
Estimated Time:45s
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Topics

IPv4 Addressing and Subnetting

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