Describe the operation and necessity of using private and public IP addresses for IPv4 addressing

Exam: Cisco 200-120 - Interconnecting Cisco Networking Devices: Accelerated (CCNAX)

IP Addressing

An IP (Internet protocol) address is aunique address assigned to a computer on an IP network. All hosts on a network have a logical ID which is called an IP Address. IP address is a software address or a logical address that contains valuable encoded information that is designed to allow hosts on one network to communicate with a host on a different network. Any device connected to IP network must have unique IP address within its network. An IP address allocation is managed by Internet Assigned Number Authority (IANA).

The internet protocol designers define an IP address as a 32-bit number, also known as Internet Protocol Version 4 (IPv4). These 32 bits are divided into four sections, referred as octets or bytes. These bytes are four sets of numbers separated by dot (.) and each set represents 8-bit number ranging from 0 to 255. You can represent an IP address in different ways:

  • Dotted-decimal as
  • Binary as 11000000.10101000.00001010.00000010

Both examples represent same IP address.

Internet Protocol essentially is the internet layer which looks at the address of each packet. It usesa routing table to decideby choosing the best path where a packet should be sent. The network address identifies each network uniquely. Every machine on the same network shares a network address as a part of its IP address. The 32-bit IP address is a structured or hierarchical address having an advantage of handling a number of addresses which count to about 4.3 billion or 4,294,967,296 in number.

IP Address Classes

Class A Address

In class A address first octet is assigned for network address and the remaining three bytes are used for the node (host) address. Class A network is defined between 0 - 127 in the first octet and uses subnet mask of by default.

These network addresses are one byte long (8 bits). The first bit of the byte is reserved and the remaining seven bits are available for addressing. You can create a maximum of 128 class A networks because the position of the remaining seven bits can be either 0 or 1. 2^7 or 128.

The usable number of network address are 128 minus 2 or 126. As we know, Class A address has three bytes reserved for the host address which means that the number of usable hosts in class A network range to 2^24 minus 2, which is actually equal to 16,777,214.

For example is a class A address where 10 is the network address and 1.5.65 is host address.

Class B Address

In class B address, the first two octets are assigned for network address and the remaining two bytes are used for the node (host) address. Class B network is defined between 128 -191 and uses a subnet mask of by default.

As it has the network address counting to two bytes so there will be a total of 2^16 combinations.But the Internet protocol designers have decided to start class B network address with the binary digit 1 and then 0, which leaves the 14 bits to manipulate, so the number of class B networks are 2^14 which is equal to 16,384 addresses.

Just like Class A, the Class B reserves 2 bytes for the host addresses. Therefore, the number of usable hosts in class B network are 2^16 minus 2 which is equal to 65,534.

For example is a class B address in which 172.16 is network address and 32.60 is host address.

Class C Address

In class C address, the first three octets are assigned for the network address and the last byte is used for the node (host) address. It can be defined between 192 -223 and uses subnet mask of by default. In this network, the position of first three bits is always binary 110, which leave 21 bits to manipulate.So, the possible number of networks in Class C are 2^21 or 2,097,152.

Coming to the hosts, class C network has 1 byte reserved for them. So, the number of usable hosts in Class C are 2^8-2 which is equal to 254.

For example is a class C address in which 192.168.102 is network address and 120 is host address.

Class D and E Address

Class D and E exist but are not used by the end users. The addresses lying between 224 and 255 are reserved for class D and E networks. Class D which ranges from 224 – 239 is used for multicast addresses while Class E ranges from 240 – 255 and is used for scientific purposes.

Private Address

Internet designers reserved a portion of IP addresses named as private address for the hosts within the organization that do not require direct access to the internet. An IP address in the private address range cannot be assigned as public address. These addresses can be used on private network, but they are not routable through the internet. The Internet Assigned Number Authority (IANA) has reserved the number of IP’s as private IP network range and these addresses are defined in RFC 1918. These IP’s are reserved for organizations that want to create internal network infrastructure.

The private IP range is defined by three address blocks:

  • This private network is Class A network ID and the valid IP’s are range from to
  • This private network is Class B network ID and the valid IP’s are range from to
  • This private network is Class C network ID and the valid IP’s are range from to


A network can be further divided into two or more networksthroughsubnetting. It allows the users to create multiple logical networks from a single Class A, B and C network. If you do not subnet then you can use only one network from class A, B or C. The benefit of subnetting is that it reduces the network traffic and optimizes the network performance.

Exam Question

Which class of IP provides a maximum of only 254 host’s addresses per network ID?

  • Class A
  • Class B
  • Class C
  • Class D
  • Class E

Answer: C

The number of usable hosts in Class C are 2^8-2 which is equal to 254.

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