Which network mask is the equivalent of 255.255.248.0?

The correct answer is B. /21. To find the CIDR equivalent of a dotted-decimal subnet mask, convert each octet to binary and count the contiguous '1's from left to right.

Junos OS Fundamentals

Question

Options

A/20
B/21
C/22
D/23

How the community answered

(48 responses)

A
8% (4)
B
73% (35)
C
4% (2)
D
15% (7)

Why each option

To find the CIDR equivalent of a dotted-decimal subnet mask, convert each octet to binary and count the contiguous '1's from left to right.

A/20

A /20 network mask would be 255.255.240.0 (11111111.11111111.11110000.00000000), which has only 20 ones.

B/21Correct

The subnet mask 255.255.248.0 translates to 11111111.11111111.11111000.00000000 in binary, which contains a total of 8 + 8 + 5 = 21 contiguous '1's from the left, thus corresponding to a /21 CIDR notation.

C/22

A /22 network mask would be 255.255.252.0 (11111111.11111111.11111100.00000000), which has 22 ones.

D/23

A /23 network mask would be 255.255.254.0 (11111111.11111111.11111110.00000000), which has 23 ones.

Concept tested: Subnet mask to CIDR notation conversion

Source: https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/ipaddr_dns/configuration/xe-3s/iad-xe-3s-book/iad-ipv4-addr-cfg.html

Topics

#CIDR notation#network mask#subnetting

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