IPv4 Addresses and Subnet Masks Explained

• A subnet mask reveals how many bits in an IP address are used for the network by masking the network portion of the IP address.
• An IP (Internet Protocol) address identifies a computer or device on a network for communication.

IPv4 Addresses

• IPv4 addresses are 32-bit numeric addresses.
• These are written as four numbers separated by periods, such as 192.168.1.0.
• Each number group separated by periods is called an octet.
• Octets range from 0 to 255.
• An IP address includes a network address/ID and a host address/ID.
• The network address is a unique number assigned to a network.
• The host address is a unique number assigned to hosts (computers, servers, tablets, routers, etc.) within a network.

Subnet Mask Function

• A subnet mask identifies which part of the IP address is the network and which is the host.
• Computers and networks interpret IP addresses and subnet masks as binary numbers (1s and 0s), bits
• Default subnet masks make it easier to identify network/host portions.
• Subnet masks with octets other than 255 require conversion to binary to identify network/host portions.
  • With a subnet mask of 255.255.224.0, the first two octets and the first three bits of the third octet of the IP address represent the network, while the remaining 13 bits are for hosts.

Binary Conversion

• Each bit in an octet is represented by a number that doubles with each step from right to left (1, 2, 4, 8, 16, 32, 64, 128).
• If a bit is '1', its represented number counts; if '0', it doesn't.
• Octets can represent numbers from 0 to 255 through the manipulation of 1s and 0s.
• To convert a decimal number to binary, use the octet chart: place 1s under the numbers in the octet chart that add up to the total and 0s elsewhere (e.g., 192 is 128 and 64).
• In a subnet mask, 1s indicate the network address portion, and 0s represent the host address portion.

Use Cases

• When the subnet mask binary digit is a 1, it indicates the position of the IP address that defines the network.
• The 1s in the subnet mask indicate the network address, while the 0s indicate the host addresses.

Network and Host

• IP addresses have network and host parts for manageability, which allows large networks to be divided into subnetworks (subnetting).
• Without subnetting, large networks experience excessive broadcast traffic, which slows them down.
• A broadcast occurs when a computer sends data to all computers on a network to locate and communicate with a specific computer.
• Routers logically break down networks into smaller networks.
• Routers help alleviate excessive traffic because broadcasts do not go past routers; broadcasts stay within a network.

Subnetting Details

• Subnetting changes the default subnet mask by borrowing bits designated for hosts to create subnets.
• Borrowing bits from the host portion of the subnet mask creates more networks.
• The more bits borrowed, the more networks are created, but the number of hosts per network decreases.
• If the default subnet mask is left as is, there is one network (254 usable hosts).
• Borrowing one bit divides the network in half (two networks with 126 hosts each).
• Borrowing two bits results in four subnets with 62 hosts each.
• Borrowing three bits yields eight subnets with 30 hosts each.
  • 4 bits = 16 subnets with 14 hosts each
  • 5 bits = 32 subnets with 6 hosts each
  • 6 bits = 64 subnets with 2 hosts each
• Borrowing 7 bits leads to 128 subnets but zero usable hosts.
• The number of networks that can be created doubles with each bit borrowed; the number of hosts per network gets cut in half with each bit.

IP Address Classes with Subnet Mask

• IP addresses and subnet masks come in five classes (A-E), but three (A-C) are for commercial use.
• The networking needs of an organization determine the IP address class required, determinable by the number in the first octet of the IP address and the default subnet mask
• Class A IP addresses can produce up to 16 million hosts.
  • The default Class A subnet mask has a large host part because three octets are used for hosts.
  • Internet service providers are an example of organizations that need many hosts.
• Class B IP addresses can produce up to 65,000 hosts and are suitable for medium to large organizations.
• Class C IP addresses can produce 254 hosts and are for small organizations and homes.

CIDR Notation

• Subnet masks can be expressed using CIDR (Classless Inter-Domain Routing), also known as slash notation.
• CIDR notation is a shorter way to write a subnet mask by counting the 1s in the subnet mask and writing it after a forward slash (e.g., /24 means 24 bits are 1s).
  • /25 indicates a subnet mask with 25 bits in length.
  • /26 means 26 bits in length.
  • /8 means 8 bits in length.