Introduction to IP addressing and Subnetting
Network devices use IP addresses and subnets to identify the source and destination of
communications and manage network addresses respectively.
IP addresses contain two parts: a network identifier and a host identifier. The network ID
specifies an area of the network where a device resides, much like an area code
identifies a section of a telephone network. The host ID labels a specific device in that
network section, similar to how a telephone number identifies a specific phone within an
area code.
Most business networks still rely on IP version 4 (IPv4) addresses, which offer about 4.3
billion unique variatiIons. Internet identities use most of these, but the newer IP version
6 (IPv6) standard provides more addresses and other benefits.
Computers work with IPv4 addresses as 32-bit binary strings. Humans usually convert
binary to a dotted decimal address, which is easier to write and understand. For
example, the binary string 11000000.00000000.00000010.00000010 converts to the IP
address 192.0.2.2. Its associated subnet mask converts from
11111111.11111111.11111111.11111100 to 255.255.255.252.
Subnet masks clarify which part of the address is the network ID and which is the host
ID. Routers, computers and network troubleshooters use IP addresses and subnet
masks to manage network traffic, which ensures information sent from one system
arrives at its destination.
IP address fundamentals
Network devices typically have the following three identities:
● Physical address. MAC address.
● Logical address. IP address.
● Hostname. Useful for humans to recognize the device.
In the case of IPv4 addresses, each host has a unique IP address on the network, and nodes
rely on it to exchange information. Data breaks down into pieces, known as packets, which then
travel across the network. Each packet contains a source and destination IP address, much like
postal service mail includes a destination and return address on the envelope.
Administrators can subdivide networks into smaller, more manageable sections. This process
lets them control the flow of network traffic, isolate traffic to increase security and limit traffic to
certain areas of the network to improve performance.
These network segments are called subnetworks, or subnets. Each subnet has a unique
identifier within the larger network ID. When administrators divide a network into parts, such as
engineering and finance, the address structure is based on numbers.
Private IP address ranges
Systems and sites on the public internet consume most of the roughly 4.3 billion
available IPv4 addresses. However, private networks can reserve some ranges for
internal use. Networks in home offices and businesses likely use one of these ranges.
The following table defines private IPv4 address ranges.
Subnet vs. subnet mask
Confusion arises between the terms subnet and subnet mask. Subnets are an isolated
network segment, which is a piece of the network that an administrator divides off.
Network administrators typically create subnets for the following reasons:
● Security. Subnets isolate traffic for privacy.
● Performance. Subnets manage traffic to reduce network congestion.
● Subnets created for security purposes prevent specific traffic types from
traversing the rest of the network, where they could be vulnerable to interception.
For example, network administrators might isolate all computers in the finance
department to a single subnet, which effectively prevents finance network
communications from moving across the entire network.
● Subnets for performance decrease competition for network access on congested
networks. If an organization's engineering department regularly transfers files
that affect network performance for other users, network administrators can
isolate the engineers to their own subnet, which reduces the effect of their file
transfers on other users. Network administrators often define multiple subnets for
both security and performance reasons.
● Subnet masks, on the other hand, play a different role. Network devices use the
subnet mask to determine which part of a host's IP address is the network ID and
which part is the host ID. Subnet masks are always used with IP addresses.
Essentially, subnets are an area of the network, while subnet masks help devices
determine the network area to which they belong.
IP address classes
IPv4 addresses divide into five classes. The primary difference between the first three
classes -- Class A, Class B and Class C -- is the number of subnets versus the number
of hosts per subnet.
The following table outlines the details between IP address classes.
Class D addresses are for a specific type of communication known as multicasting,
which lets a host transmit thousands of data streams to other devices. Class E also isn't
used with standard IPv4 networking. Instead, IP address researchers use Class E
addresses for experimental and developmental purposes. This classification system is
known as classful addressing. Classful addressing relies on rigid divisions between the
network ID and host ID. These divisions always occur at the eighth, 16th or 24th bits in
the address.
This design often wastes addresses, however. For example, Class B networks provide
65,000 host IDs, and Class C networks offer only 254 host IDs. If a network has 2,000
devices, and administrators select Class B, they could waste around 63,000 addresses.
Classless addressing, on the other hand, lets network administrators define network IDs
at any point for more efficient use of the available addresses. For example, to satisfy a
network with 2,000 host IDs, administrators can use a subnet mask of 255.255.248.0/21
to provide 2,048 total host IDs. The extra number at the end of the subnet mask is
called a Classless Inter-Domain Routing (CIDR) notation, which specifies a number of
bits in the subnet mask. This method lets admins add more IP addresses without using
a different class.
The following table shows examples of subnet masks, their corresponding CIDR
notations and the number of total IP addresses per subnet.
Troubleshoot IP addresses and subnet masks
Network administrators often face two common IP address troubleshooting scenarios:
misconfiguration and an inability to communicate with an essential network service.
Network administrators can manually configure some devices, such as servers, routers
and printers, with an IP address, subnet mask and other network settings. However, any
typographical error in these settings prevents the device from communicating correctly
on the network.
One of the first network troubleshooting steps is to verify these settings, including the
subnet mask. Network administrators should update subnet masks with valid settings if
they're incorrect.
The following table lists commands that display IP address settings on three popular
OSes.
