What is a packet?

A packet is the unit of data transmission at the network layer (L3). It is in the form of packets that data is transmitted between different networks.

Short version: a packet is data packaged according to the rules of the network layer so that it can be routed.

---

#Why a packet is needed

The data link layer can only deliver data inside a single network. A packet is needed to:

• transmit data through several networks
• indicate the IP address of the sender and the receiver
• allow routers to decide where to send the data next

---

#What a packet consists of

#1. Packet header (IP header)

The service part, which contains:

• the source IP address
• the destination IP address
• routing information
• auxiliary fields (for example, TTL, protocol type)

Routers analyze only the header, not the data itself.

---

#2. Payload

Inside a packet is data from the transport layer:

• a TCP segment or UDP datagram
• application data (indirectly, inside the segment)

The network layer:

• does not check the content of this data
• does not guarantee its delivery

---

#What is important to understand

• a packet is L3
• a packet does not contain MAC addresses
• a packet can be:
  • transmitted
  • fragmented
  • dropped by a router

---

#Relationship with other layers

• transport layer → segment
• network layer → packet
• data link layer → frame
• physical layer → bits

During transmission:

• the packet stays the same
• frames change at every segment of the network

---

#What a packet does NOT do

Important for an interview A packet:

• does not guarantee delivery
• does not control speed
• does not track transmission order
• does not correct errors

That is handled by the transport layer.

---

#In short, for an interview

A packet is a unit of network layer data containing an IP header and a payload, used for transmitting data between networks and for routing.