Computer Networks: Subnet Masks and VLSM

Variable Length Subnet Masks (VLSM)

• Conserves IP addresses by creating subnets of different sizes
• Breaks up a network into smaller subnets with varying sizes (e.g., /24, /30)
• Suitable for links between offices that require only a few IP addresses

Subnet Masks

• Breaks a network into smaller subnets
• Slash 30 network uses 30 bits for the network and 2 bits for the host, resulting in 4 IP addresses

IP Addresses

• Assigned to devices and referred to as host addresses
• Unicast traffic involves a device sending traffic to only one other device
• Broadcast traffic involves a device sending a message to every device in the local network
• Broadcast IP is the last IP in the local network, with all host bits turned on

Network and Broadcast Addresses

• Network address has all host bits set to 0 (e.g., 172.16.2.0/24)
• Broadcast address has all host bits set to 1 (e.g., 172.16.2.255)
• Network and broadcast addresses can be calculated using the IP address and subnet mask

VLSM Calculations

• Utilizes the "magic number" method to calculate subnets and broadcast addresses
• Calculates the subnet mask (e.g., slash 20 has 20 ones in the subnet mask)
• Calculates the third octet in the subnet mask with a mixture of ones and zeros
• Counts by the value of the subnet mask to find the start of the network and broadcast addresses

Default Gateway and Routing

• Devices use a default gateway (router) to send traffic to other networks
• Default gateway is the IP address of the local router
• Routers forward traffic between networks but do not forward broadcast messages

Multicast

• Enables devices to opt-in to receiving certain traffic
• Utilizes special IP addresses (Class D, 224.0.0.0 to 239.255.255.255)
• Devices can opt-in to receive traffic sent to a multicast IP address
• Routers forward multicast traffic, allowing it to reach other networks

IP Address Management

• IANA manages IP addresses and assigns blocks to Regional Internet Registries (RIRs)
• RIRs assign addresses to organizations, which then assign them to devices
• Private IP addresses (RFC 1918) are reserved for local use and cannot be routed on the internet
• Network Address Translation (NAT) allows devices to access the internet using public IP addresses

Obtaining IP Addresses

• Devices can obtain IP addresses statically (manually configured) or dynamically (using a DHCP server)
• DHCP servers assign IP addresses from a pool and ensure uniqueness
• Automatic Private IP Addressing (APIPA) is used by Windows devices when a DHCP server is not available

IP Addressing Basics

• IP addresses identify devices on a network and consist of four numbers (octets) separated by dots, with each octet being an 8-bit value ranging from 0 to 255 for easier human understanding.
• An IP address represents two addresses: the device address and the network address.

IPv4 and IPv6

• IPv4 is the older version of IP addressing, still widely used today.
• IPv6 is the newer version, introduced to address the growing need for IP addresses.

Classful IP Addressing

• The original method used the first octet to represent the network, and the remaining three octets for hosts, but was limited to only 255 possible networks and eventually replaced.

Class A, B, and C Networks

• Class A networks use 1 octet for the network and 3 octets for hosts.
• Class B networks use 2 octets for the network and 2 octets for hosts.
• Class C networks use 3 octets for the network and 1 octet for hosts.

IP Address Classes

• Class A: 1.0.0.0 to 126.0.0.0, with 128 networks and 16,777,216 hosts per network.
• Class B: 128.0.0.0 to 191.255.0.0, with 16,384 networks and 65,536 hosts per network.
• Class C: 192.0.0.0 to 223.255.255.0, with 2,097,152 networks and 256 hosts per network.

Subnet Mask and CIDR

• Subnet mask determines which part of an IP address is the network and which part is the host.
• CIDR is a method of subnetting that allows for more efficient use of IP addresses.
• Subnet mask is written in the same format as an IP address, with bits set to 1 for the network and 0 for hosts.

Subnetting

• Subnetting breaks a large network into smaller subnets for more efficient IP address use and better network organization.
• Subnet mask determines the number of subnets and hosts per subnet.

CIDR Notation

• CIDR notation is a shorthand way of writing the subnet mask, e.g., 172.16.0.0/24, where /24 represents the subnet mask.

Classful vs. Classless Networking

• Classful networking uses fixed classes (A, B, and C) to determine the network and host parts of an IP address.
• Classless networking uses CIDR and subnetting for more flexible and efficient IP address use.
• Although classful networking has been replaced, understanding the concept is essential for exams and everyday networking.