IP: Internet Protocol

Launched in 1974, by Vint Cerf and Bob Kahn.
IP is the Layer 3 protocol supporting Layer 4 protocols such as:
- TCP
- UDP
- GRE
- ESP
- … and many more
Other protocols that do a similar job to IP:
- IPX/SPX
- CLNS
“Layer 3” and “Layer 4” refer to the OSI Layer Model

The OSI Layer Model (in short)

OSI = Open Systems Interconnection
Standard of the ISO (International Organisation for Standardization): https://www.iso.org/about-us.html
Download it from here: https://standards.iso.org/ittf/PubliclyAvailableStandards/index.html (search for the string “7498-1:1994”)

Quoting the standard

” 6.1.4. The highest is the Application Layer and it consists of the application-entities that cooperate in the OSI Environment. The lower layers provide the services through which the application entities cooperate.

6.1.5. Layers 1-6, together with the physical media for OSI provide a step-by-step enhancement of communication services. The boundary between layers identifies a stage in this enhancement process at which an OSI service standard is defined while the functioning of the layers is governed by OSI protocol standards. ”

The Seven OSI Layers

Layer 7: Application   HTTP, HTTPS, IMAP, WebSockets
Layer 6: Presentation  MIME, SSL/TLS, ASCII, MPEG
Layer 5: Session       TCP Sockets, named pipes, RPC
Layer 4: Transport     TCP, UDP, GRE, ESP
Layer 3: Network       IP, ICMP, IPSEC, OSPF, RIP
Layer 2: Data Link     ARP, VLAN (802.1q), PPP, IS-IS
Layer 1: Physical      RJ45, RS-232, 802.3 PHY (10BASE-T,
                       100BASE-T, 1000BASE-T), DOCSIS,
                       DWDM

Tools for converting between bases

https://www.rapidtables.com/convert/number/decimal-to-binary.html
macOS Calculator (Programmer mode)
linux, macOS: bc (apt install bc / brew install bc)

$ echo "obase=2;192" | bc
11000000
$ echo "ibase=2;11000000" | bc
192
$

IP Addresses

IP addresses are used to identify Layer 3 interfaces on an IP network
They have a fixed length of 32 bits,
usually written as four groups of eight bits with a dot between each eight:

   11000000.10101000.00000000.00000001

except converted to decimal:

   192.     168.     0.       1

That ^ is called a “dotted quad”
You can also express it as a 32-bit integer, if you want to:

3232235521

IPv4 and IPv6

IPv4 addresses: 32 bits long
32 bits = 4,294,967,295 combinations of on or off, 1 or 0
=> only 4,294,967,295 IPv4 addresses (minus reservations)
… but we have more devices than that
tricks for sharing IPv4 addresses: NAT (Network Address Translation)
IPv6 addresses: 128 bits long
128 bits = 340,282,366,920,938,463,463,374,607,431,768,211,456 (340 trillion trillion trillion) combinations of on or off
=> 340,282,366,920,938,463,463,374,607,431,768,211,456 IPv6 addresses
There are maybe 400 billion stars in the milky way.

Network Masks 1

IP addresses are often written along with a network mask.

        192.168.0.1   255.255.255.0

or,

        192.168.0.1/24

32 bits long (or 128 bits long for IPv6).

        255.255.255.0 = 11111111.11111111.11111111.00000000

Clarifies the address as “network part” and “host part”

Network Masks 2

Same “network” part => communicate directly, no Router
Different “network” part => must use a Router to communicate
No gaps allowed in the bits in the network mask!

IP Address Example: 203.33.18.1/24

203.33.18.1/24

   203  .   33   .   18   .    1     /24
11001011.00100001.00010010.00000001  /24

the first 24 bits are the network part
the remaining (32 - 24 = 8) bits are the host part

   203  .   33   .   18   .    0     /24
11001011.00100001.00010010.00000000  /24
========.========.========                  <= network part, written as 203.33.18.0/24

   203  .   33   .   18   .    1     /24
11001011.00100001.00010010.00000001  /24
                           ========        <= host part, written as 203.33.18.1/24

IP Address Example: 192.168.56.43/16

192.168.56.43/16

   192  .  168   .   56   .   43     /16
11000000.10101000.00111000.00101011  /16

the first 16 bits are the network part
the remaining (32 - 16 = 16) bits are the host part

   192  .  168   .    0   .    0     /16
11000000.10101000.00000000.00000000  /16
========.========                          <= network part, written as: 192.168.0.0/16


   192  .  168   .   56   .   43     /16
11000000.10101000.00111000.00101011  /16
                  ========.========        <= host part, written as: 192.168.56.43/16

IP Address Example: 150.101.5.6/23

150.101.5.6/23

   150  .  101   .   5   .    6     /23
10010110.01100101.00000101.00000110 /23

the first 23 bits are the network part
the remaining (32 - 23 = 9) bits are the host part

   150  .  101   .   4   .    0     /23
10010110.01100101.00000100.00000000 /23
========.========.=======                  <= network part, written as 150.101.4.0/23


   150  .  101   .   5   .    6     /23
10010110.01100101.00000101.00000110 /23
                         =.========        <= network part, written as 150.101.5.6/23

Finding your own IP address(es)

Linux: ip a
Windows: ipconfig
macOS: ifconfig

Questions

What is your local IP address?
What is your network mask?
How many unique IP addresses are available in your local network?

linux: ip a | windows: ipconfig | macos: ifconfig

Handy tool: ipcalc

S1E2 - IPv4 Addressing