1. Help! Tell me about Computer Data!

2. How to recognize data sizes
0x 4A BC DE F2
Byte
Half word
Word
One Word = 32 bits = 8 hexadecimal characters
One Halfword = 16 bits = 4 hexadecimal characters
One Byte = 8 bits = 2 hexadecimal characters
One Nibble = 4 bits = 1 hexadecimal character

3. IPv4 Packet Header showing bit layout
Version
(0-3)
Hlenth
(4-7)
Service Type
(8-15)
Total Length
(16-31)
Datagram Identification
(0-15)
Flags
(16-19)
Fragment Offset
(20-31)
Time to Live
Protocol
Header Checksum
Source IP Address
Destination IP Address
Type
Code
Checksum
Identifier
Sequence Number
Data

4. IPv4 Packet Header: Interpreting data
dc 039c b c0a8 0004
c0a c
a 6b6c 6d6e 6f
Version
(0-3)
Hlenth
(4-7)
Service Type
(8-15)
Total Length
(16-31)
Datagram Identification
(0-15)
Flags
(16-19)
Fragment Offset
(20-31)
Time to Live
Protocol
Header Checksum
Source IP Address
Destination IP Address
Type
Code
Checksum
Identifier
Sequence Number
Data dc
039c 2000
Notice that each table row is 32 bits = 1 word = 8 hexadecimal characters…
Allocate the hex characters to their proper row, then to their proper field within row.

5. IPv4 Packet Header: Solution for first 2 rows
dc 039c b c0a8 0004
c0a c
a 6b6c 6d6e 6f
Version =4
Hlenth =5
Service Type
=00
Total Length
=05dc
Datagram Identification
(0-15) = 039c
Flags
=B0010
Fragment Offset
(20-31) = 0000
Time to Live
Protocol
Header Checksum
Source IP Address
Destination IP Address
Type
Code
Checksum
Identifier
Sequence Number
Data dc
039c 2000
Notice that each table row is 32 bits = 1 word = 8 hexadecimal characters…
Allocate the hex characters to their proper row, then to their proper field within row.

6. Converting Special Fields
IP Address
Flags
Example: 0x1a2b 3c4d
32 bits are separated:
8bits. 8bits . 8bits . 8bits
0x1a. 0x2b. 0x3c. 0x4d
Convert to decimal:
0x1a= x2b=43
0x3c= x4d=77
IP address:
Example: 0x53 21
Convert to binary:
Bit 16: (0101)
Bit 15: (0101)
Bit 14: (0101)
Bit 13: 1
Bit 12: 0