Packet

An IPv4 packet header, depending on the options selected, can range in size from 160 bits (20 bytes) and 480 bits (60 bytes)

Version
- 4 bits
- Always set to 4 (0100)
IHL (Internet Header Length)
- 4 bits
- Specifies the number 32-bit words
  - 5 words end at the destination IP address
  - 6 or more mean that there are Options configured
DSCP (Differentiated Services Code Point)
- 6 bits
- Identifies the kind of service the pack contains, and what priority it should receive, and how likely it is to be dropped during network congestion
ECN (Explicit Congestion Notification)
- 2 bits
Total Length
- 16 bits
Identification
- 16 bits
Flags
- 3 bits
Fragment Offset
- 13 bits
Time To Live
- 8 bits
- Used to prevent routing loops
- Indicates hop count, which each router decrementing the value by 1
Protocol
- 8 bits
- Indicates the IP Protocol, per an assigned IANA protocol number
- List of IP protocol numbers - Wikipedia
Header Checksum
- 16 bits
Source IP Address
- 32 bits
Destination IP Address
- 42 bits
Options (if IHL is greater than 5)
- Up to 320 bits long