IP Addressing and Subnetting Basics

Overview

This lecture covers how IP addresses are divided into network IDs, subnet IDs, and host IDs, with a focus on subnetting, subnet masks, and their binary representation.

IP Address Structure and Subnetting

• An IP address is a 32-bit number divided into network ID and host ID.
• Subnetting takes bits from the host ID and assigns them to the subnet ID.
• All three IDs (network, subnet, host) are represented within the same 32-bit IP address.
• Core routers use the network ID for routing datagrams to the correct network.

Subnet Masks and Their Role

• Subnet IDs are determined using a subnet mask, a 32-bit number like an IP address.
• A subnet mask contains a sequence of ones (masking the network and subnet ID) followed by zeros (for the host ID).
• Comparing an IP address and subnet mask in binary reveals which bits belong to each section.
• Example: The subnet mask 255.255.255.0 translates to 24 ones and 8 zeros.

Calculating Subnets and Hosts

• The part of the address corresponding to mask ones is for the subnet ID; zeros indicate host ID bits.
• Subnet size is dictated by the number of zero bits in the subnet mask.
• With 255.255.255.0, the last octet (8 bits) is for host IDs, supporting 256 values (0-255).
• Typically, 0 is not used (network address), and 255 is reserved for broadcast, so only 1-254 can be assigned to hosts.

Subnet Mask Short Notation

• Subnet masks can also be written in slash notation (e.g., /27 for 27 ones).
• Example: 255.255.255.224 equals /27, providing 5 host ID bits (32 addresses).
• Both decimal and slash notations are common and should be understood.

Key Terms & Definitions

• IP Address — A 32-bit number uniquely identifying a device on a network.
• Network ID — The part of the IP address identifying the network.
• Host ID — The part of the IP address identifying the device within a network.
• Subnet ID — Identifies a subnetwork within a main network, taking bits from host ID.
• Subnet Mask — A 32-bit mask showing which bits are network/subnet and which are host.
• Broadcast Address — The highest address in a subnet, used for sending to all hosts.

Action Items / Next Steps

• Practice converting IP addresses and subnet masks to binary.
• Learn how to identify network, subnet, and host IDs from given IP and subnet mask.
• Review how to calculate available hosts per subnet for different mask lengths.