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IP Subnet (IP Addressing and Subnetting)

4/18/22, 1:20 AM Subnetting

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Learning Topic

Subnetting is the practice of subdividing a network into several smaller networks.

For the initial design for the TCP/IP protocols (long before the internet was widely used),

there were three network sizes, class A, B, and C, and every network had to be in one of

these classes.

Network Address

Number of

Number of Hosts
in Each Network

Class A (X.0.0.0) –

126 16,777,214

Class B (X.Y.0.0) 128.0.0 –

16,384 65,534

Class C (X.Y.Z.0) –

2,097,152 254

However, with the growth of online activity, there are not enough IP addresses for

networks, and the addresses need to be used more efficiently. Even the smallest Class C

may be too big for some networks and thus wastes precious IP addresses.

This led to the practice of subnetting. The idea is to create a network just big enough to

host the systems on that network.

An IPv4 IP address is 32 bits long. It is usually written as 4 octets (w.x.y.z), where each

octet can be a number between 0 and 255. The 32-bit address consists of network and

host bits. For example, a Class A address will have 8 bits for network bits and 24 bits for

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host bits.

This is also where a subnet mask comes into play. A subnet mask will inform the systems

which bits are network bits and which are host bits. For example, a subnet mask for a

Class A address would be (or 11111111.00000000.00000000.00000000 when

written in binary notation). Notice the 1s in the subnet mask are used to identify that the

first 8 bits in the IP address are network bits, and the 0s signify the host bits. This is also

why a Class A address is sometimes written as X.0.0.0/8, where the “/8” is used in placed

of the subnet mask.

For example, suppose a university is given a Class B IP address ( with the

network part of 16 bits, and the remaining 16 bits of the 32-bit IP address left for host

addresses within the network. It is described by the CIDR notation of and

allows for 2 local hosts. The number of usable hosts is actually 2 – 2, because the first

and the last number are used for network and broadcast addresses.

However, even within its large local network, the university may want to have separate

local subnetworks associated with its various departments. This can be achieved by


Given a single local network with n bits for network address and h=32-n bits for host

addresses within the network, subnetting calls for borrowing bits from the host part to

designate a subnet part within the network. If we borrow b bits, the subnet part will allow

for 2 subnets.

The network mask for the network with subnets is extended by b bits.

Because there are h-b bits left in the host part of the address, the number of hosts within

each subnet is 2 .

If our example university wants to have no more than 64 departments, it can borrow 6

bits from the host part of its address space for the subnet part for each department,

allowing for up to 64 subnets. (2 = 64). The network and subnet part of IP addresses will

be of the length 16+6 = 22, leaving 16-6 = 10 bits for the host addresses within each

department’s subnet, which allows for 2 – 2 = 1022 usable hosts per department.

The subnet mask will be 11111111.11111111.11111100.00000000, which is

255.255.252 (/22 in the CIDR notation). The third octet is 252 because it is 255 (all eight

1’s which is 2 -1) minus missing two first powers of 2 (2 and 2 , which adds to 3).

The usable host addresses within each subnet are:

16 16





8 0


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Source: UMGC

Note that 25 in binary is 00011001, and 126 in binary is 01111110. They are both

written in full 8-bits format (thus the zeros in front of the number).

The green sections are the original network bits. They are 16 bits long because we started

with a Class B network.

The yellow sections are the 6 bits that we “borrowed” from the host bits. These will

eventually become part of the network bits.

The red sections are what is left of the host bits. We started with 16 and with 6 bits

borrowed, we are left with 10 bits.

As long as routers in the local network know the subnet mask, they will forward the

incoming packets to the proper subnet.

The choice for the number of bits borrowed for the subnet part is a compromise between

the desired maximum number of subnets and maximum number of allowable hosts within

each of these subnets (since the subnet and hosts part sizes need to add to the whole

network’s host part size). If you want s subnets, for the number of bits borrowed, you pick

a number b, which yields the smallest 2 greater than s. If you want to have up to a

specific number of hosts within subnets, the remaining number of bits in the host part

must allow for it.


See the following resources for more information:




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All links to external sites were verified at the time of publication. UMGC is not responsible for the validity or integrity

of information located at external sites.

IP Subnet (IP Addressing and Subnetting)

4/18/22, 1:24 AM Classless Interdomain Routing (CIDR)

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Learning Topic

Classless Interdomain Routing (CIDR)
Classless Interdomain Routing (CIDR) is a method to use IP addressing without being

restricted to the original grouping of IPv4 (Class A, B, and C). Using CIDR, you can create

a network (or subnetwork) and size it appropriately based on the needs.

For example, without CIDR, if you have a network with 500 hosts or systems,

you cannot use a Class C network because Class C is limited to 254 hosts, so

you are forced to use a Class B network, which leads to a waste of IP addresses.

Also, prior to CIDR, when specifying a network address, you will need to also provide the

subnet mask. For example:

Network address:

Subnet mask:

You need both to inform the size of the network.

With CIDR notation, instead of specifying the subnet mask, you note the number of bits

used for the network hosts. For example, the same network as above would be written as:

Network address:

For more information, review the following article:

A Linux networking guide to CIDR notation and configuration


© 2022 University of Maryland Global Campus

All links to external sites were verified at the time of publication. UMGC is not responsible for the validity or integrity

of information located at external sites.

IP Subnet (IP Addressing and Subnetting)

Transmission Control Protocol (TCP)

Transmission Control Protocol (TCP) is the protocol that is responsible for the reliable delivery of messages on the internet. It is imperative to have a general understanding of how the transport layer creates and maintains a connection between two networked systems and how the exchange between the two systems occurs.


Sliding Windows

Data to be transmitted on a network is broken down into smaller packets. These packets are then sent to the destination one packet at a time. Due to the nature of internet routing, some packets may take a different route to the destination. Thus, the receiving system may receive the packets out of order or could even lose some packets.

Sliding windows is the concept of how these packets are sent to the network. The sender doesn’t just send one packet and wait for confirmation of receipt before sending the next packet. Instead, the sender will continuously send packets up to the window size (the amount of data that can be “packed” into a packet before being sent), while also watching for the confirmation message to come in.

8.2   Sliding Windows


Internet Assigned Numbers Authority (IANA)


A Linux networking guide to CIDR notation and configuration


IP Subnet (IP Addressing and Subnetting)


– Please cite your work in your responses

– Please use APA (7th edition) formatting 

– All questions and each part of the question should be answered in detail (Go into depth)

– Response to questions must demonstrate understanding and application of concepts covered in class, 

– Use in-text citations and at LEAST 2 resources per discussion from the school materials that I provided to support all answers. 

– No grammatical errors; Complete sentences are used. Proper formatting is used. Citations are used according to APA

The use of course materials to support ideas is HIGHLY RECOMMENDED

– Lastly, Responses MUST be organized (Should be logical and easy to follow)

Start Here

Step 3: Investigate Internet Transport—TCP

As robust as the IP protocol is, it does not perform the actual transmission of the data. In this step, you will investigate the network protocol called 
Transmission Control Protocol (TCP)
, responsible for creation, reliability of delivery, and proper assembling of data packets. 

In addition to IP, TCP is also widely used on the internet, especially for any network communication where it is essential to confirm receipt of the transmission. Many of the network protocols used to implement cloud computing use both TCP and IP. You will review TCP’s workings and discuss them in your final technical report. 

In general, there is no guarantee that a data packet will reach its destination. Packets can get lost or corrupted during transmission, and there are network applications where you need assurance that the packets have reached their destination. To achieve reliability, TCP establishes connections between communicating hosts, using port numbers to refer to applications on these hosts. Then, packets are created, sequenced, transmitted, acknowledged, and retransmitted if missing or containing errors. Finally, at the destination, they are reassembled into the original messages. 

To synchronize the flow of packets between sender and receiver, and avoid packet congestion in case of varying speeds, TCP uses 
sliding windows
 for packets remaining in processing at a given time, at both the sender and receiver ends. 

In the next step, you will look into subnetting BallotOnline’s IP addresses.

Step 4: Review Advantages of IP Subnetting

One of the drawbacks of IPv4 is the maximum number of network devices it can support. IPv4 addressing uses a 32-bit network address. This allows for 232,, or a little over 4 billion devices. However, today there are significantly more devices on the internet. Even though the more robust IPv6 version has been introduced and efforts are under way to assure wide adoption, IPv4 is still widely used.

One method used to more efficiently use the IPv4 network addresses is a technique to optimize the addresses by splitting them into network addresses and host addresses within designated networks. You will need to take advantage of IP address splitting so that you can efficiently use and allocate the IPv4 network addresses that have been assigned to BallotOnline.

For a given large network, rather than addressing all the hosts using the host part of the address, 
 allows for splitting the network into several smaller ones by borrowing the host part bits and adding them to the network bits. It supports efficient management of local networks composed of multiple LANs. In this step, you will investigate subnetting conventions and discuss them in your final report in order to lay ground for the use of subnets by BallotOnline.

As the network engineer for BallotOnline, you know that subnetting a network into several smaller and variable-sized networks will be best for the organization’s needs. BallotOnline has been assigned a network address block by the 
Internet Assigned Numbers Authority (IANA)

Because BallotOnline has several offices around the world, you will need to subnet this network address to assign just the right number of addresses to each office location based on the number of systems at that office location (the network address assigned by IANA and the number of offices along with their sizes will be provided in the announcement section of this class at the beginning of Week 2).

Your task is to create the appropriate number of subnets with the right size to optimize the network.

Your technical report should include 
Classless Interdomain Routing (CIDR)
 notation, which allows for specifying a variable-length network as part of the IPv4 address, leaving the rest for addressing hosts within the network.



Here are the details for Step 4:

Ballot Online has been assigned the following block of Class B IP addresses:

It allows for up to 2 to the power of 32-19 which is 13, i.e. 8192 IP addresses.

Ballot Online wants to open 4 field offices, with up to 2000 IP network hosts (desktops and other externally accessible networked devices) each.

Design the 4 subnets for these field offices. For each subnet, show the subnet mask, network and broadcast addresses in x.x.x.x decimal format as well as 32-bit binary (0 or 1) bitmaps, and the range of usable host addresses within the subnet.

PS. Here is a link to a good video explaining subnetting https://www.youtube.com/watch?v=jRszJ-NPwtI