What is a subnet mask?
A subnet mask is a number used to calculate the IP composition.
An IP consists of 2 parts: Network Id and Host Id, and the subnet mask serves to distinguish these two parts.
Because IPv4 is a 32-bit number, the subnet mask is also a 32-bit number.
The subnet mask is a concept in IPv4 that no longer exists in IPv6.
web identity
Suppose, my host IP is 192.168.1.100, how do I determine if another IP 192.168.1.200 is on the same network segment as my host?
The same network segment
If you use the default subnet mask, which is 255.255.255.0, the IP and subnet mask by bit and arithmetic will give you the network identifier.
As follows, the network identifier of the first IP is
192.168.001.100
& 255.255.255.000
--------------------
= 192.168.001.000The network identifier for the second IP is.
192.168.001.200
& 255.255.255.000
--------------------
= 192.168.001.000Their network identifiers are the same, both are 192.168.1.0, so they are inside the same network segment.
Different network segments
In another case, if we use a mask of 255.255.255.192, then press the bitwise sum operation.
The network identifier for the first IP is.
192.168.001.100
& 255.255.255.192
--------------------
= 192.168.001.64The network identifier for the second IP is.
192.168.001.200
& 255.255.255.192
--------------------
= 192.168.001.192At this point they have different network identifiers, so they are no longer on the same network segment and cannot interoperate.
subnet assignment
Because we want to be able to isolate the hosts on the LAN by this mechanism.
Assuming our router uses network segment 192.168.1.0, there are probably a total of 200 or so computers connected to it.
Then, if we want them to be connected to each other, then just use the default mask and all IPs in the range 192.168.1.0-255 will be on a subnet and will all be connected to each other.
If we want to segregate them, for example, in a company where development, finance, marketing, and human resources each use their own IP segment, then we need to segment the IP, and here we can divide it into 4 segments:
- Paragraph 1 development segment:
192.168.1.0 - 63 - Segment 2 for finance:
192.168.1.64 - 127 - Segment 3 for the market:
192.168.1.128 - 191 - Para. 4 Manpower segment:
192.168.1.192 - 255
And how are subnet masks assigned?
The 192.168.1.0 segment has a total of 256 addresses, divided into 4 subnets, each with 64 IP addresses.
64 is the 6th power of 2, and the subnet mask should consist of 26 1s and 6 0s, converted to decimal as 255.255.255.192, which corresponds to binary as.
11111111.11111111.11111111.11000000The IP address range of each department's computer is set, and the subnet mask must be set to 255.255.255.192, and the computers within each department can be networked normally after the settings are completed, and the computers between different departments cannot be connected directly.
Network segment and mask representation
We have divided 4 segments above, how can we represent these segments in a simple way?
Generally, the first address plus the mask bit is used to represent the entire IP segment, as follows.
- Paragraph 1 development segment:
192.168.1.0 - 63, expressed as:192.168.1.0/26 - Segment 2 for finance:
192.168.1.64 - 127', expressed as192.168.1.64/26' - Paragraph 3 market segment:
192.168.1.128 - 191, expressed as:192.168.1.128/26 - Para. 4 Human segment:
192.168.1.192 - 255, expressed as:192.168.1.192/26
where 26 indicates the number of 1s in the mask, which is equivalent to the mask 255.255.255.192.
Note that the first and last addresses of each segment have special uses and are not available to users; the first address is the segment address, as in 192.168.1.0 above, and the last is the broadcast address, as in 192.168.1.63 above.