Question 1

What is a subnet mask and CIDR prefix?

Accepted Answer

A subnet mask splits an IPv4 address into a network part and a host part. The CIDR prefix is just a shorthand for how many bits belong to the network — /24 means the first 24 bits, which is the same as the mask 255.255.255.0. The remaining bits are available for hosts, so a /24 has 8 host bits and 256 total addresses.

Question 2

How many usable hosts are in a subnet?

Accepted Answer

For most subnets the usable host count is 2 to the power of the host bits, minus 2 — one address is reserved as the network address and one as the broadcast address. A /24 therefore has 256 total addresses and 254 usable hosts. The exceptions are /31, which gives 2 usable addresses for point-to-point links under RFC 3021, and /32, which is a single host.

Question 3

What is the difference between the network and broadcast address?

Accepted Answer

The network address is the first address in the subnet, with all host bits set to 0, and it identifies the subnet itself. The broadcast address is the last address, with all host bits set to 1, and traffic sent to it reaches every host on the subnet. Neither can normally be assigned to a device, which is why they are excluded from the usable host range.

Question 4

Which IP address ranges are private?

Accepted Answer

RFC 1918 reserves three private ranges that are not routed on the public internet: 10.0.0.0/8, 172.16.0.0/12 and 192.168.0.0/16. The calculator also flags other special ranges such as 127.0.0.0/8 (loopback), 169.254.0.0/16 (link-local / APIPA) and 100.64.0.0/10 (carrier-grade NAT).

Question 5

What is a wildcard mask?

Accepted Answer

A wildcard mask is the bitwise inverse of the subnet mask — where the subnet mask has a 1, the wildcard has a 0, and vice versa. For a /24, the mask 255.255.255.0 has the wildcard 0.0.0.255. Wildcard masks are commonly used in router access control lists and OSPF configuration on Cisco-style devices.

#	Network	Usable range	Broadcast
1	192.168.1.0/26	192.168.1.1 – 192.168.1.62	192.168.1.63
2	192.168.1.64/26	192.168.1.65 – 192.168.1.126	192.168.1.127
3	192.168.1.128/26	192.168.1.129 – 192.168.1.190	192.168.1.191
4	192.168.1.192/26	192.168.1.193 – 192.168.1.254	192.168.1.255