Subnetting An IPv4 Network

Network Segmentation

Reasons for Subnetting

Subnettingis the process of segmenting a network into multiple smaller network spaces called subnetworksor subnets.

• Large networks must be segmented into smaller subnetworks, creating smaller groups of devices and services to:
  • Control traffic by containing broadcast traffic within each subnetwork.
  • Reduce overall network traffic and improve network performance.

Communication Between Subnets

• A router is necessary for devices on different networks and subnets to communicate.
• Each router interface must have an IPv4 host address that belongs to the network or subnet that the router interface is connected.
• Devices on a network and subnet use the router interface attached to their LAN as their default gateway.

IP Subnetting is FUNdamental
The Plan

Subnetting an IPv4 Network
Basic Subnetting

Borrowing Bits to Create Subnets Borrowing 1 bit 21= 2 subnets
Borrowing 1 Bit from the host portion creates 2 subnets with the same subnet mask
Subnet 0 Network 192.168.1.0-127/25 Mask: 255.255.255.128	Subnet 1 Network 192.168.1.128-255/25 Mask: 255.255.255.128

Subnets in Use

Subnetting Formulas

Calculate number of subnets
Calculate number of hosts

Creating 4 Subnets

Borrowing 2 bits to create 4 subnets. 2² = 4 subnets

Creating Eight Subnets

Borrowing 3 bits to Create 8 Subnets. 2³ = 8 subnets

Determining the Subnet Mask
Subnetting Based on Host Requirements

Two considerations when planning subnets:

• Number of subnets required
• Number of host addresses required

Formula to determine number of usable hosts: 2^n-2

• 2ⁿ(where nis the number of remaining host bits) is used to calculate the number of hosts.
• -2 (The subnetworkID and broadcast address cannot be used on each subnet.)

Subnetting Network-Based Requirements

Calculate the number of subnets: 2^n(where n is the number of bits borrowed) Subnet needed for each department.

Subnetting To Meet Network Requirements

Balance the required number of subnets and hosts for the largest subnet. Design the addressing scheme to accommodate the maximum number of hosts for each subnet. Allow for growth in each subnet.

Traditional Subnetting Wastes Addresses

Traditional subnetting–Uses the same number of addresses is allocated for each subnet	Subnets that require fewer addresses have unused (wasted) addresses; for example, WAN links only need two addresses.

Variable Length Subnet Masks (VLSM)

	The variable-length subnet mask (VLSM) or subnetting a subnet provides more efficient use of addresses.
	VLSM allows a network space to be divided in unequal parts.
	Subnet mask varies, depending on how many bits have been borrowed for a particular subnet.
Network is first subnetted, and then the subnets are resubnetted.

Basic VLSM

VLSM in Practice

Using VLSM subnets, the LAN and WAN segments in example below can be addressed with minimum waste.

Each LANs will be assigned a subnet with /27 mask.

Each WAN link will be assigned a subnet with /30 mask.

VLSM Chart

Design Considerations for IPv6

Planning to Address the Network

Allocation of network addresses should be planned and documented for the purposes of:

• Preventing duplication of addresses
• Providing and controlling access
• Monitoring security and performance

Client addresses –Usually dynamically assigned using the Dynamic Host Configuration Protocol (DHCP).
Sample Network Addressing Plan

Subnetting Using the Subnet ID

An IPv6 Network Space is subnettedto support hierarchical, logical design of the network

IPV6 Subnet Allocation

Subnetting into the Interface ID

IPv6 bits can be borrowed from the interface ID to create additional IPv6 subnets.