As we’ve explained about IPs’ structure before,all IPv4 addresses have 4 octets. According to this fact, Network ID and Host ID are defined.
All devices get an IP when they want to connect to a network. So, imagine you have 2 devices and their IP, how do you know if they are in the same network or not? the answer is : Network ID.
Network ID is the fixed octets of your IPs. for example: supposing that 172.16.2.1 and 172.16.3.1 are 2 IPs. The first two octets are identical and fixed for both IPs. So, both of these IPs are in the same network and their network ID is 172.16.
Host ID is the variable octets in IPs. This means that considering the example IPs above (172.16.3.1 , 172.16.2.1) , 3.1 and 2.1 are the Host IDs of our IPs.
Class A: the first octet of these class of IP is fixed. This means that the first octet is Network ID and all the other octets are host ID.The octet range for this class is 1 to 126.
Class B: the first two octets of these class of IP are fixed. This means that the first two octets are Network ID and the other two octets are host ID. The octet range for this class is 128 to 191.
Class C: the first three octets of these class of IP are fixed. This means that the first three octets are Network ID and all the last octet is host ID. The octet range for this class is 192 to 223.
Class D: This class of IP is a special class which we use for multi-casting. Multi-casting is used when we want to send specific packets to specific stations. The octet range for this class is 224 to 239.
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