"Class A" IPv4 addresses are for very large networks. Please note below important points about Class A network IP addresses.
- First octet of a "Class A" network IP address is used to identify the "Network part" and the remaining three octets are used to identify a host uniquely within that network.
- The default structure of a Class A IP address is Network.Host.Host.Host
- The left-most bit [also called as high-order bit or most-significant bit] of the left most octet of a "Class A" network is reserved as "0".
I strongly suggest you to visit and learn below lessons before you continue learning more about Class A networks and Class A IP addresses.
- IPv4 Addresses
- What is subnet mask
- What is network address
- Binary Decimal and Hexadecimal numbers and conversions
- Five layered TCP/IP model
- Network Layer [Layer 3]
- IPv4 Protocol, IPv4 header and fields of IPv4 header
- Media Access Control [MAC] addresses
- Limited broadcast address
- Directed broadcast address
- What is Loopback address
Structure of a Class A IP address
The left-most octet of a Class A network IP address belongs to the network part and remaining three octets belongs to the host part. The structure of a Class A IP address is Network.Host.Host.Host. The default subnet mask of a Class A IP address is 255.0.0.0.
Please refer below image.
The number of networks and the number of hosts per class can be derived by this formula −
When calculating hosts' IP addresses, 2 IP addresses are decreased because they cannot be assigned to hosts, i.e. the first IP of a network is network number and the last IP is reserved for Broadcast IP.
Class A Address
The first bit of the first octet is always set to 0 [zero]. Thus the first octet ranges from 1 – 127, i.e.
Class A addresses only include IP starting from 1.x.x.x to 126.x.x.x only. The IP range 127.x.x.x is reserved for loopback IP addresses.
The default subnet mask for Class A IP address is 255.0.0.0 which implies that Class A addressing can have 126 networks [27-2] and 16777214 hosts [224-2].
Class A IP address format is thus: 0NNNNNNN.HHHHHHHH.HHHHHHHH.HHHHHHHH
Class B Address
An IP address which belongs to class B has the first two bits in the first octet set to 10, i.e.
Class B IP Addresses range from 128.0.x.x to 191.255.x.x. The default subnet mask for Class B is 255.255.x.x.
Class B has 16384 [214] Network addresses and 65534 [216-2] Host addresses.
Class B IP address format is: 10NNNNNN.NNNNNNNN.HHHHHHHH.HHHHHHHH
Class C Address
The first octet of Class C IP address has its first 3 bits set to 110, that is −
Class C IP addresses range from 192.0.0.x to 223.255.255.x. The default subnet mask for Class C is 255.255.255.x.
Class C gives 2097152 [221] Network addresses and 254 [28-2] Host addresses.
Class C IP address format is: 110NNNNN.NNNNNNNN.NNNNNNNN.HHHHHHHH
Class D Address
Very first four bits of the first octet in Class D IP addresses are set to 1110, giving a range of −
Class D has IP address range from 224.0.0.0 to 239.255.255.255. Class D is reserved for Multicasting. In multicasting data is not destined for a particular host, that is why there is no need to extract host address from the IP address, and Class D does not have any subnet mask.
Class E Address
This IP Class is reserved for experimental purposes only for R&D or Study. IP addresses in this class ranges from 240.0.0.0 to 255.255.255.254. Like Class D, this class too is not equipped with any subnet mask.