IP Packet Header Analysis from First 8 Bits

The question asks to interpret the first 8 bits of an IP packet header: $0100 0101$. These bits contain crucial information about the IP version and the header length.

Deconstructing the IP Header Fields

The first 8 bits of an IPv4 header are structured as follows:

• The first 4 bits represent the IP Version.
• The next 4 bits represent the Internet Header Length (IHL).

Determining IP Version

The first 4 bits are $0100$.

• Converting binary $0100$ to decimal: $0 \times 2^3 + 1 \times 2^2 + 0 \times 2^1 + 0 \times 2^0 = 4$.
• An IP version of 4 indicates that this is an IPv4 packet, not IPv6.

Calculating Internet Header Length (IHL)

The next 4 bits are $0101$.

• Converting binary $0101$ to decimal: $0 \times 2^3 + 1 \times 2^2 + 0 \times 2^1 + 1 \times 2^0 = 5$.
• This value, 5, is the IHL.

Calculating Total Header Size

The IHL specifies the header length in units of 32-bit words. Since 32 bits equal 4 bytes, the total header size in bytes is calculated as:

Evaluating the Options

• Option 1: The IP version of the packet is 6. Incorrect. The version is 4 ($0100$).
• Option 2: The Time To Live ($TTL$) value is 5. Incorrect. The $TTL$ field is located later in the header and is not determined by the first 8 bits.
• Option 3: The packet is malformed and will be rejected. Incorrect. An IHL of 5 is valid for IPv4, resulting in a standard header size of 20 bytes.
• Option 4: The total number of bytes in the header is 20 bytes. Correct. As calculated, the header length is $5 \times 4 = 20$ bytes.

Conclusion

Based on the analysis of the first 8 bits ($0100 0101$), the IP version is 4 and the Internet Header Length (IHL) is 5. This means the total header size is 20 bytes.