---
id: 1777370971-DYNV
aliases:
  - IPv4 Header
tags: 
    - Cisco
    - CCNA
---

# IPv4 Header


## OSI Model - [[PDU]]s

Application Data
↓
Data + L4 Header = Segment
↓
Segment + L3 Header = Packet
↓
Packet + L2 Header + L2 Trailer = Frame

We will focus on the Packet for this lesson

## IPV4 header

### Version Field

length: 4 bits (half of on octet)
- identifies the version of IP used.
- IPv4 = 4 (0b0100)
- IPv4 = 6 (0b0110)

### Internet Header Length (IHL)

length: 4 bits (half of on octet)

- The final field of the IPv4 header (Options) is variable in length, so this field
is necessary to indicate the total length of the header
- identifies the lenght of the header in **4 byte increments**
- Value of 5 = 5*4 bytes = 20 bytes
- minimum value is 5 (= 20 bytes)
  - **Minimum IPv4 Header Length = 20 bytes**
- Maximum value is 15 (15*4 bytes = 60 bytes)
  - **Maximum IPv4 Header Length = 60 bytes**

### DSCP field

- Length: 6 bits
- Differentiated Services Code Point
- Used for Qos (Quality of Service)
- Used to prioritize delay-sensitive dta (streaming voice, video, etc..)

### ECN field

Explicit Congestion Notification
- Length: 2 bits
- Provides end-to-end (between two endpoints) notification of network congestion 
**without dropping packets**
- Optional feature that requires both endpoints, as well as the underlying network 
infrastructure, to support it.

### Total Length field

Length: 16 bits
- Indicates the total lenght of the packet (L3 header+ L4 segment)
- Measured in bytes (**not 4-byte increments like IHL**)
- Minimum value of 20 (=IPv4 header with no encapsulated data)
- Maximum value of 65,535 (maximum 16-bit value)

### Identification field

Length: 16 bits
- if a packet is fragmented due to being too large, this field is used to identify
which packet the fragment belongs to
- All fragments of the same packet will have their own [[IPv4]] 
header with the same value in this field
- Packets are fragmented if larger than the MTU (Maximum Transmission Unit)
- The MTU is usually 1500 bytes
- Fragments are reassembled by the receiving host

### Flags field

Length: 3bits

- Used to control/identify fragments.
- Bit 0: Reserved, always set to 0
- Bit 1: Don't Fragment (DF bit), used to indicate a packet that should not be fragmented
- Bit 2: More Fragments (MF bit), set to 1 if there are more fragments in the packet,
set to 0 for the last fragment

*Unfragemented packets will always have their MF bit set to 0*

### Fragment Offset field

Length: 13 bits

- Used to indicate the position of the fragment within the original, unfragmented IP packet
- Allows fragmented packets to be reassembled even if the fragments arrive out of order

### Time to live field

Length: 8 bits
- A router will drop a packet with a TTL of -
- Used to prevent infinite loops
- Originlly designed to indicte the packet's maximum lifetime in second
- In practice, indicates a 'Hop count': each time the packet arrives at a router, 
the router decreases the TTL by 1
*Recommended default TTL is 64*

### Protocol field

Length: 8 bits

- Indicates the protocol of the encapsulated L4PDU
- Value of 6: [[TCP]]
- Value of 17: [[UDP]]
- Value of 1: [[ICMP]]
- Value of 89 : [[OSPF]] dynamic routing protocol

### Header Checksum field

Lenght 16bits

- A calculaed checksum used to check for errors in the IPv4 header.
- When a router receives a packet, it calculates the checksum of the header 
and compares it to the one in this field of the header
- if they do not match the router drops the packet
- Used to check for errors only in the IPV4 header.
- IP relies on the encapsulated protocol to detect errors in the encapsulated data
- Both TCP and UDP have their own checksum fields to detect errors in the encapsulated data

### Source/Destination IP address fields 

Length 32 bits(each)

- Source IP Address = IPv4 address of the sender of the packet
- Destination IP Address = IPv4 address of the intended receiver of the packet

### Options fields

Length: 0 - 320 bits

- Rarely used.
- If the IHL field is greater than 5, it means that Options are present




## Review

- IPv4 Packet structure
- Fields of the IPv4 header