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M03 IPv4 Addresses, Subnetting and ARP

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Internet Protocol version 4

Lecture Videos



The Internet Protocol is designed for use in interconnected systems of packet-switched computer communication networks. The internet protocol provides for transmitting blocks of data called datagrams from sources to destinations, where sources and destinations are hosts identified by fixed length addresses. The internet protocol also provides for fragmentation and reassembly of long datagrams, if necessary, for transmission through "small packet" networks.

Since its design in 1970 - 1980 the usage of Internet Protocol (IP) has exploded. The designers of IPv4 decided to use a 32 -bit field for addressing devices (with a theoretical maximum of 2^32 = 4 294 967 296 devices).

They never suspected that there (ever) would be a network that would have that many computers attached to it. And here we are today. Running out of IPv4 addresses. Because of that decision.

What exactly is IPv4 exhaustion?

"IPv4 exhaustion" doesn't mean that the Internet will stop working.

Instead, IPv4 exhaustion is the term used to describe when there will be no more unallocated IPv4 addresses available. But this will happen is several stages, and not necessarily in the order below:

  • IANA exhausted its IPv4 free pool (3 February 2011)
  • RIRs exhaust their unallocated pools
  • Expanding networks (ISPs, businesses, etc) exhaust their pools of unused addresses

  • Source: APNIC

The Internet Protocol occupies Layer 3 of the OSI Model.


The IPv4 currently amounts up to 70 - 80 percent of the world's Internet traffic, with the updated version IPv6 (Internet Protocol version 6) slowly, but surely taking over.

Internet Header Format

Every packet has a header. This is what an IPv4 packet header looks like.

IPv4 header


IPv4 header wireshark



A field reserved to indicate version of the header. Either 4 or 6.

IHL - Internet Header Length

Points to the beginning of the data carried by the header. And so this field also indicates the end/length of the header.

Type of Service

The field is used to assure Quality of Service to packets whilst they travel through the data networks.

Total Length

What is the length of the datagram (including header & data).


A value set in the header to aid the receiver in reassembling.


Bits (true/false) that indicate can the packet be fragmented during transport. And if the received fragment is the last one.

Fragment Offset

Informs where in the whole datagram this current packet belongs to.

Time to Live

If this field is zero, this datagram cannot be forwarded. Once sent typically the value is 255 and is decreased by one per each routing decision.


Indicates the next protocol carried by this packet. Be it TCP, UDP, ICMP or something else. IANA lists these protocols

Typical values on this course:

Value Protocol
17 UDP

Header Checksum

A verification field that the header in intact and hasn't suffered errors during transmit.

Source Address

The IPv4 address of the sender. Please see Addressing devices

Destination Address

The IPv4 address of the destination. Please see Addressing devices


Options field can be present in datagrams. They have to be transmitted if present.


Padding is used to verify that the Header is divisionable by 32 bits.

Addressing devices

Lecture Video from this point forward

The 32 -bit address space

The address space is represented (to us, people) by four decimals with dots in between. The most classic example would be

Unfortunately computers do not process decimal numbers, but binaries so the same address in binary would be.

1100 0000 . 1010 1000 . 0000 0000 . 0000 0001

And the same in hexadecimal format.


Decimal format is the most common form. Hexadecimals can be seen in e.g. Wireshark captures, but rarely IP addresses are discussed with hexadecimals. Binary form is relevant in subnetting calculations.


The idea that a Network is divided into subnetworks (or "subnets") is taken into account in addressing.

So we have a network that is divided into subnets that contain addresses. So the idea is as seen in the following figure


And to fill that idea with actual IP addresses brings us to:


Something new that is visible is the subnet mask represented by / 8, / 16 and / 24 in the drawing.

These are the basic subnet sizes of IPv4. The subnet mask declares: how many devices/addresses can fit into the subnet.

The Classful classes of IPv4 are:

Class Start of the binary sequence Protocol Addresses Network Mask Amount of devices
A 0 8 bits of net, 24 bits of host - / 8 2^24 = 16 777 216
B 10 16 bits of net, 16 bits of host - / 16 2^16 = 65 536
C 110 24 bits of net, 8 bits of host - / 24 2^8 = 256
D 1110 - - - Multicast
E 110 - - - Experimental


Lecture Video from this point forward

As stated earlier, the IPv4 address space is exhausting. This called for an strategy to reserve addresses by allowing more dynamic use of subnet masks.

Further information can be read from the declaring RFC.

Classless Inter-domain Routing (CIDR): The Internet Address Assignment and Aggregation Plan

This memo discusses the strategy for address assignment of the existing 32-bit IPv4 address space with a view toward conserving the address space and limiting the growth rate of global routing state.

But at this point in the course, we just need to acknowledge that subnet masks can be anything between /0 to /32. More of this in Subnetting.

Reserved Addresses

Within the address spaces, but still not permitted to use:

  • Loopback IP addresses are between -
    • Handled by the operating system, the most notable being
  • Link-local Addresses are between -
    • If there is a failure in contacting a DHCP Server, the client chooses a random link-local address

Public IPv4 vs Private IPv4 addresses

RFC 1918 - Address Allocation for Private Internets

Dated February 1996;

"The Internet has grown beyond anyone's expectations. Sustained exponential growth continues to introduce new challenges. One challenge is a concern within the community that globally unique address space will be exhausted."

This meant that it was ideal to re-use addresses where direct connection to the Internet wasn't necessary. These addresses could be Network Address Translated - NATted (later on the course) to public addresses.

Private address spaces were declared as

  • - ( / 8)
  • - ( / 12)
  • - ( / 16)

Addressing Windows 10

Here is an example of manual addressing of a Windows 10 machine.


Note that you have used to dynamic addressing, but you can only automate things once you understand how they are done manually.

DHCP - Dynamic Host Control Protocol is learned later on the course!

Windows knows this as the term Obtain an IP address automatically to activate DHCP client.

Given the figure, note the fields

  • IP address: an IP address given to the device for use
  • Subnet mask: the subnet size in DEC format, would be /24 in slash format
  • Default gateway: note that the term gateway lives on here rather than router, this is the gateway out of the subnet... similar to a door out of a classroom being the default gateway to access the world


  • Preferred DNS server: devices need to translate domain names to IP addresses, DNS is discussed later on the course
The settings can be found in Windows 10 by pressing the WIN button and typing ncpa.cpl


  1. From the Network Connections, choose the Adapter you want to configure by right click + Properties

  2. Select Internet Protocol Version 4 (TCP/IPv4) and click Properties

Linux world has same principles, but the graphical interface changes a little bit and are behind different windows/selection boxes



Lecture Video from this point forward

Between the subnets are gateways (or routers). They are represented in topology drawings with the logo:


They make the decision to forward traffic between subnets introduced earlier.


Routing at the simplest form is to look at the Destination Address of an IP packet and decide what interface it has to be sent out of.

Gateways / Routers

The earlier RFCs talked about gateways, but now-a-days they are called Routers.

Routers work on the OSI Layer 3 by forwarding packets.


What does a Router actually look like?

Routers found at home are usually (partly) managed by the ISP, but owned by the customer. They integrate a lot of functionalities in one box.

  • OSI Model Layer 1: modem (4G/5G to WLAN to Cabled Ethernet),
  • OSI Model Layer 2: switch
  • OSI Model Layer 3: router
  • ...



Routers in industrial use are typically more heftier e.g. Cisco ASR 1002-HX Router below



Routing Table

Given the topology above, Routing Table is a database that the router collects and maintains.

Adapter IP Network Mask Interface
1 / 24 Ethernet 1
2 / 16 Ethernet 2
Warning: Real routing tables are a bit more complicated, but the basic idea is the same


If the router receives a packet destined for, it can calculate the subnet it belongs to and send it out of the correct interface. By looking at the table above, it would be Ethernet 1.


Lecture Video from this point forward

The Math

Subnet Masks

The idea of subnet masks is to declare how many of the address bits are the network section (n) and how many are the host/device section (h). In the following example the mask is /24, so

  • 24 pieces of n bits
  • 8 bits of h bits

So if the address is = 1100 0000 . 1010 1000 . 0000 0000 . 0110 0100

And the subnet mask is = 1111 1111 . 1111 1111 . 1111 1111 . 0000 0000

If we count all the 1 bits of the subnet mask, we come to the total sum of 24. Thus 24 times 1 bits is represented by /24.

To help humans visualize this; now imagine all the 1 bits are represented by n bits (network bits) and all the 0 are h bits as follows,

nnnn nnnn . nnnn nnnn . nnnn nnnn . hhhh hhhh

What is the subnet?

Now you can use the n bits (or just the subnet mask directly in binary format) and place it next to the given address.

1100 0000 . 1010 1000 . 0000 0000 . 0110 0100 (typically the address goes first)

1111 1111 . 1111 1111 . 1111 1111 . 0000 0000 (then the subnet mask)

Now we look at the first bits of both of the rows.

1 = input A

1 = input B

Both are ran through and AND -gate.


Truth table of AND.

Input A Input B Output
0 0 0
0 1 0
1 0 0
1 1 1

Thus with the inputs above 1and 1, the output is 1.

When we go through all of the bits, we get the result of

1100 0000 . 1010 1000 . 0000 0000 . 0000 0000 =

Which is called the subnet address. Typically the size of the subnet is represented after the address, e.g.

How many hosts can fit into the subnet?

Now you can use the h bits to get the "size of the subnet". It answers to the question; "how many hosts can the subnet fit?"

We had 8 h bits. We cycle through all of them.

binary decimal purpose
0000 0000 0 Subnet address
0000 0001 1 1st host address
0000 0010 2 2nd host address
0000 0011 3 3rd host address
0000 0100 4 4th host address
.... .... . ................
1111 1100 252 252th host address
1111 1101 253 253th host address
1111 1110 254 254th host address
1111 1111 255 Broadcast Address

Thus if we take out the Subnet Address and Broadcast address we get 254 usable addresses.

Otherway to calculate the "size of the subnet" is to have formula: 2^h - 2

2^8 - 2 = 254, where -2 comes from the subnet address and broadcast address being unavailable for host use.

Supernetting vs Subnetting

Lecture Video from this point forward

This is called Variable Length Subnet Mask - VLSM

Supernetting is to increase the "size of the subnet". The goal is to increase the amount of hosts that fit into the subnet.

/23 -> 2^9 - 2 = 510 hosts

/22 -> 2^10 - 2 = 1022 hosts

/21 -> 2^11 - 2 = 2046 hosts

Note worthy is that this takes away the bits from the subnet mask (the n bits decrease and the h bits increase). E.g.

/22 =

nnnn nnnn . nnnn nnnn . nnnn nnhh . hhhh hhhh =

1111 1111 . 1111 1111 . 1111 1100 . 0000 0000 =

As the host bits increase to 10, we get a supernet of 2^10 - 2 = 1022

Subnetting is to decrease the "size of the subnet". The goal is to decrease the amount of hosts that fit into the subnet.

/25 -> 2^7 - 2 = 126 hosts

/26 -> 2^6 - 2 = 62 hosts

/27 -> 2^5 - 2 = 30 hosts

Noteworthy is that this increases the bits of the subnet mask (the n bits increase and the h bits decrease).

/26 =

nnnn nnnn . nnnn nnnn . nnnn nnnn . nnhh hhhh =

1111 1111 . 1111 1111 . 1111 1111 . 1100 0000 =

As the host bits decrease to 6, we get a subnet of 2^6 - 2 = 62

Design & Documentation

In IPv4 the design is to use as little IPv4 addresses as possible, but still room for possible growth.

Typically this means:

  • links between devices are subnetted with /30
  • local area networks with /24 (because of tradition of class C subnets).
    • Smaller are suggested if plausible to conserve public IP address space

Still other varying subnets might occur, and thus calculation is a basic knowhow of an engineer in IT.

With this design, a typical network would look like:


Address Resolution Protocol - ARP

Lecture Video from this point forward

ARP is the glue between OSI Model Layer 3 and Layer 2. This means that there are multiple occasions when an MAC Address is needed for an IP address and vice versa.

Everytime a network device needs to know this information (e.g. to forward traffic). ARP comes into play.

The following examples happen when needs to send traffic to presented in the following diagram.


When devices are given IP addresses, they only know their own IP and MAC address. This can be seen below when checking the IP address of lubuntu.

lubuntu@lubuntu-VirtualBox:~$ ip addr
1: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP group default qlen 1000
    link/ether 08:00:27:0e:55:17 brd ff:ff:ff:ff:ff:ff
    inet brd scope global eth0
       valid_lft forever preferred_lft forever
    inet6 fe80::a00:27ff:fe0e:5517/64 scope link 
       valid_lft forever preferred_lft forever
Ethernet adapter Ethernet:

   Connection-specific DNS Suffix  . :
   Description . . . . . . . . . . . : Realtek PCIe GBE Family Controller
   Physical Address. . . . . . . . . : BC-AE-C5-24-74-1D
   DHCP Enabled. . . . . . . . . . . : Yes
   IPv4 Address. . . . . . . . . . . :
   Subnet Mask . . . . . . . . . . . :
   Lease Obtained. . . . . . . . . . : Wednesday 3. February 2021 8.10.21
   Lease Expires . . . . . . . . . . : Wednesday 3. February 2021 13.10.21
   Default Gateway . . . . . . . . . :
   DNS Servers . . . . . . . . . . . :

The machine needs to contact its default gateway (or the router connecting the subnet) to forward traffic out of its own subnet. So ARP works between these devices.


When the device is addressed, it is given a gateway address. This can be checked by looking at the routing table of the computer.

lubuntu@lubuntu-VirtualBox:~$ route -v
Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
default         UG    0      0        0 eth0    *        U     1      0        0 eth0

Note here that the gateway term lives on. Even though people are talking about routers!

C:\Users\sahka>route print
~~~ tulostetta poistettu ~~~
IPv4 Route Table
Active Routes:
Network Destination     Netmask          Gateway        Interface   Metric       25    On-link   281  On-link   281  On-link   281
Persistent Routes:

Note here that the gateway term lives on. Even though people are talking about routers!

The information on the computer now is, and this is called the ARP table:

IP address MAC address 08:00:27:0e:55:17 ?

Now the computer needs to send an ARP message to the whole subnet (a broadcast) to find out what is the MAC address of This is represented in the figure below.

Who has Tell


The answer can be seen below. is at 08:00:27:87:12:ef


When this message transaction is done. The computer now knows the ARP table (checked by the command arp -a)

lubuntu@lubuntu-VirtualBox:~$ arp -a
? ( at 08:00:27:87:12:ef [ether] on eth0
C:\Users\sahka>arp -a

Interface: --- 0xe
  Internet Address      Physical Address      Type           08-00-27-87-12-ef     dynamic

Thus the ARP table is now fulfilled and traffic can continue.

IP address MAC address 08:00:27:0e:55:17 08:00:27:87:12:ef

Continue to the Exercises

E03 Calculating Subnets

E04 Addressing Devices

Self-reflect the material with a small quiz?

Data Networks Quiz - M03 IPv4 Addresses, Subnetting and ARP

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This course and its materials are written by Karo Saharinen and licenced by Attribution-NonCommercial-NoDerivatives 4.0 International (CC BY-NC-ND 4.0) license.

Creative Commons -licence