Subnetting - Subnet Masks

  • Network IDs, Host IDs, and Subnet IDs

  • Network ID: Identifies the network itself.

  • Host ID: Identifies individual devices (hosts) within the network.

  • Subnet ID: Introduces a division within the network to enable better management and utilization of IP addresses.

  • IP Address Basics

  • An IP address is a 32-bit number.

  • Without subnetting, bits are allocated solely for Network ID and Host ID.

  • Subnetting allows some bits from the Host ID to represent the Subnet ID, improving routing efficiency.

  • Routing Process

  • Core routers utilize Network IDs to direct data to the appropriate gateway for further routing.

  • The gateway router uses additional data to route the datagram to its final destination.

  • The Host ID is used by the last router to deliver the data to the specific machine.

  • Subnet Masks

  • A subnet mask is also a 32-bit number, written as four octets in decimal format.

  • It defines which portion of an IP address is the Subnet ID and which portion is the Host ID.

  • Commonly mischaracterized as a 'magic number,' it requires understanding for effective use.

  • Binary Representation

  • Each part of an IP address is an octet (8 bits).

  • For example, the number 9 in binary is represented as 00001001 to fill the octet.

  • Subnet masks consist of a string of ones followed by zeros, indicating which bits are relevant for identifying the Subnet ID.

  • Example of Subnet Mask

  • Common subnet mask: 255.255.255.0

    • Translates to 24 ones followed by 8 zeros.

  • Differentiates Subnet ID (ones) from Host ID (zeros).

  • Calculating Subnet Size

  • The size of a subnet is determined by the subnet mask.

  • With a mask of 255.255.255.0, the last octet is used for Host IDs, yielding potential addresses 0-255.

  • However, 0 is not usable, and 255 is typically reserved for broadcast, leaving effective host addresses as 1-254.

  • Hence, a subnet allows for 254 usable addresses, but it’s common to mention the full number of addresses (256) for simplicity.

  • Non-Standard Subnet Masks

  • Masks can also use non-octet boundaries, such as 255.255.255.224 (27 ones + 5 zeros).

  • This allows 32 total addresses (but typically 30 usable addresses).

  • Subnet Notation

  • Subnet masks can be represented in shorthand notation: /27 for the example stated above.

  • A complete address could be expressed as 9.100.100.100/27, linking IP and subnet mask concisely.

  • Understanding both traditional and shorthand notations is essential.