Subnet Masks 관련

How IPv4 Works – A Handbook for Developers

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OraThe Internet Protocol version 4 (IPv4) is one of the core protocols of standards-based internetworking methods in the Internet and other packet-switched networks. IPv4 is still the most widely deployed Internet protocol. Google’s IPv6 Statistics show...

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How IPv4 Works – A Handbook for Developers

OraThe Internet Protocol version 4 (IPv4) is one of the core protocols of standards-based internetworking methods in the Internet and other packet-switched networks. IPv4 is still the most widely deployed Internet protocol. Google’s IPv6 Statistics show...

Another way to express the network prefix is by using a subnet mask, like so:

255.255.0.0

When converted to binary, 255 in decimal equals eight 1s in binary – so all bits are on. So if you translate this mask into binary, you get:

11111111 11111111 00000000 00000000

In other words, 16 bits are on, which means a network prefix of 16 bits. Both conventions (CIDR notation and subnet masks) are used very frequently.

16-bit subnet mask address
(Source: BriefVid)

With CIDR, an address can reside in different networks given different network prefixes, or subnet masks. If you consider the same example address with a different prefix, say that of 8 bits – both additional addresses would belong to the same network, as they all share the first 8 bits – 200.

How would you present a network prefix of 8 bits as a subnet mask? You need the first 8 bits to be on, so that means 255 in decimal, and the remaining bits are off, resulting in this subnet mask:

255.0.0.0

8-bit subnet mask address
(Source: BriefVid)

What happens if you use a network prefix of 24 bits? First, how would you express that as a subnet mask? You need 24 bits to be on, so that is 3 times 8 bits to be on, resulting in:

255.255.255.0

24-bit subnet mask address
(Source: BriefVid)

Now, neither of the additional addresses reside within the same network as the example address, as they don't share its network ID of 200.8.3.

CIDR
(Source: BriefVid)

Note that network prefixes do not have to represent full bytes. For example, you can use a network prefix of 12 bits, or 11 bits, or 22 bits. When the prefix length isn't a multiple of 8, the subnet mask will have a value other than 0 or 255 in one of its positions.

This addresses the issue regarding the startup company. If a startup has 300 employees, they'd need to get a 23-bits network ID, leaving 9 bits for hosts within their networks. This means 2^9, or 512 addresses, which should be sufficient.