Subnetting (Part 1)
Day 13
Subnetting
The IANA (Internet Assigned Numbers Authority) assigns IPv4 addresses/networks to companies based on their size. For example, large companies may receive a Class A or Class B network, while a small company might receive a Class C network. But it led to many wasted IP addresses. E.g. when using a point-to-point network - a network connecting only two points ( two offices, branches, cities, etc), only 2 addresses are assigned to these two points, one for the network, and one for the broadcast; even if we use Class C network we waste 252 addresses.
The IETF (Internet Engineering Task Force) introduced CIDR (Classless Inter-Domain Routing) in 1993 to replace the classful addressing system. With CIDR, larger networks were allowed to be split into smaller networks, allowing greater efficiency. These smaller networks are called subnetworks or subnets.
Class C subnetting
/25
The network portion of the address is extended into the first bit of the last octet. And the mask in dotted decimal is now 255.255.255.128. There are now 7 bits in the host portion of the address, so the number of usable addresses is 2^7-2=126.
/26
The mask in dotted decimal is written as 255.255.255.192. The number of usable addresses is 62.
/27
The mask is 255.255.255.224. The number of usable addresses is 30.
/28
The mask is 255.255.255.240. The number of usable addresses is 14.
/29
The mask is 255.255.255.248. The number of usable addresses is 6.
/30
The mask is 255.255.255.252. The number of usable addresses is 2 - perfect for point-to-point networks. 203.0.113.0/30 includes the address range of 203.0.113.0 - 203.0.113.3 first address being the network address, the next two are usable addresses, and the last one is a broadcast address. The remaining addresses in the 203.0.113.0/24 address block are available to be used in other subnets.
/31
The mask is 255.255.255.254. The number of usable addresses is 0. However, this subnet mask can be used for point-to-point networks because it consists of only 2 addresses and there is no need for the network and broadcast addresses. For any other cases, this would be a problem since it leaves no usable addresses after subtracting the network and broadcast addresses.
/32
The mask is 255.255.255.255. This mask cannot be used here and usually is not used at all for configuration. However, there are some use cases, e.g. when creating a static route to one specific host.
Here is a chart summarizing dotted decimal subnet masks and their equivalent in CIDR notation.
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