What is a port?

A port is a logical number at the transport layer (L4) used to identify a specific application or process on a device.

Simply put: a port is needed to determine which exact application on the computer the data is intended for.

---

#Why a port is needed

The IP address answers the question:

which computer to send the data to

The port answers the question:

which program on that computer to pass the data to

Without ports:

• data would reach the device
• but the system would not know which application to hand it to

---

#How a port is used in practice

Data transmission always uses two ports:

• source port - which application the data was sent from
• destination port - which application it is intended for

Example:

• browser → port 443 (HTTPS)
• mail client → port 25 (SMTP)

---

#Port range

A port is a number from 0 to 65535.

#Main ranges:

#1. Well-known ports (0-1023)

• standard services
• usually require administrator privileges

Examples:

• 80 - HTTP
• 443 - HTTPS
• 22 - SSH
• 25 - SMTP

---

#2. Registered ports (1024-49151)

• applications and services
• often used by servers and programs

---

#3. Dynamic / Ephemeral ports (49152-65535)

• temporary ports
• usually used by clients
• assigned automatically by the OS

---

#Ports and protocols

Important for the interview. Ports exist only together with a protocol:

• TCP port 80 ≠ UDP port 80
• TCP and UDP have separate port spaces

---

#What is important to remember

• a port is not a physical connector

• a port is a logical concept

• a port belongs to the transport layer

• the combination:

  IP + port + protocol

uniquely identifies a connection