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Chapter 8 Subnetting IP networks (Subnetting an IPv4 Network (Network…
Chapter 8 Subnetting IP networks
Subnetting an IPv4 Network
Network segmentation
Broadcast Domains
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.
Locate services
– 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
Octet Boundaries
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.
Classless Subnetting
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.
Magic number
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.
IPv6 subnetting is not concerned with conserving address space. The subnet ID includes more than enough subnets. IPv6 subnetting is about building an
addressing hierarchy
based on the number of subnetworks needed.
Subnetting Using the Subnet ID
Note: Subnetting into the 64 bit Interface ID (or host portion) is also possible but it is rarely required.
Structure of an IPv6
global unicast address
Global routing prefix
: This is the prefix, or network, portion of the address that is assigned by the provider. Typically, Regional Internet Registries (RIR) assign a /48 global routing prefix to ISPs and customers
Subnet ID
: Used by an organization to identify subsets within its site
Interface ID
: This equivalent to the host portion of an IPv4 address. The term interface ID is used because a single host may have multiple interfaces, each having one or more IPv6 address
Example
With IPv6 the WAN link subnet will not be subnetted further. Although this may “waste” addresses, that is not a concern when using IPv6.
When configuring IPv6 on a router remember to
enable IPv6 routing on the router
:
R1(config)#ipv6 unicast-routing
