Endianness

<aside> 📖 Little Endian: least significant byte is at lowest address

Makes most sense from an addressing/computational standpoint

</aside>

<aside> 📖 Big Endian: most significant byte is at lowest address

Makes most sense to a human reader

</aside>

Network Byte Order

• Different processors have different endianess
  • x86 is little endian, while ARM is big endian
• To interoperate, they need to agree how to represent multi-byte fields
• Network byte order is big endian

Protable Code

You have to convert network byte order values to your host order

Helper functions: htons(), ntohs(), htonl(), ntohl()

Packet Formats

The block of text on the left is taken verbatim from Request for Comments (RFC) 791, which specifies the IP protocol, version 4, or IPv4.

• For historical reasons, Internet specifications are written in plain ASCII text.
• The top shows the bits from 0 to 31 -- packets are written 4 bytes wide.
• Since IPv4 has 5 rows of required fields, this means that an IPv4 header is at least 20 bytes long.