Chapter 8 Subnetting IP networks
Chapter 8 Subnetting IP networks
Subnetting an IPv4 Network
In an Ethernet LAN, devices use broadcasts to:
Locate other devices
– A device uses Address Resolution Protocol (ARP) which sends Layer 2 broadcasts to a known IPv4 address on the local network to discover the associated MAC address.
– A host typically acquires its IPv4 address configuration using the Dynamic Host Configuration Protocol (DHCP) which sends broadcasts on the local network to locate a DHCP server.
Switches propagate broadcasts
out all interfaces except the interface on which it was received
Routers do not propagate broadcasts.
Therefore, each router interface connects a broadcast domain and broadcasts are only propagated within its specific broadcast domain.
A broadcast domain is a logical division of a computer network, in which all nodes can reach each other by broadcast at the data link layer.
Problems with Large Broadcast Domains
A problem with a large broadcast domain is that these
hosts can generate excessive broadcasts and negatively affect the network
broadcast traffic resulting in:
Slow network operations due to the significant amount of traffic it can cause
Slow device operations because a device must accept and process each broadcast packet
The solution is to reduce the size of the network to create smaller broadcast domains in a process called subnetting. These smaller network spaces are called subnets.
Broadcasts are only propagated within the smaller broadcast domains.
Reasons for Subnetting
Reduces overall network traffic and improves network performance
Enables an administrator to implement security policies such as which subnets are allowed or not allowed to communicate together.
group devices and services into subnets
IPv4 subnets are created by using one or more of the host bits as network bits. This is done by extending the subnet mask to borrow some of the bits from the host portion of the address to create additional network bits
Networks are most easily subnetted at the octet boundary of /8, /16, and /24.
Subnets can borrow bits from any host bit position to create other masks.
For instance, a /24 network address is commonly subnetted using longer prefix lengths by borrowing bits from the fourth octet. This provides the administrator with additional flexibility when assigning network addresses to a smaller number of end devices.
Subnetting Based on Host Requirements
There are two considerations when planning subnets:
the number of host addresses required for each network
number of hosts = (2^bits of the host portion)-2
the number of individual subnets needed
number of subnets = 2^bits borrowed
Variable Length Subnet Masks
VLSM allows a network space to be divided into unequal parts. With VLSM, the subnet mask will vary depending on how many bits have been borrowed for a particular subnet, thus the “variable” part of the VLSM.
Subnetting an IPv6 Network
There are two types of assignable IPv6 addresses. An IPv6 link-local address is never subnetted because it exists only on the local link. However, an IPv6 global unicast address can be subnetted.
The IPv6 global unicast address normally consists of a /48 global routing prefix (provided by the RIR, Regional Internet Registries), a 16 bit subnet ID, and a 64 bit interface ID.
Subnetting Using the Subnet ID