Which of the following fields is not part of the IP packet header?

The Class of Service (CoS) is not a standard field in the IP packet header. Instead, it is more commonly associated with the data link layer, particularly in the context of Ethernet frames where it is used for prioritizing traffic. In contrast, the other fields listed are indeed part of the IP packet header.

Time to Live (TTL) serves as a counter to limit the lifespan of a packet in the network, preventing it from circulating indefinitely. The Header Length specifies the size of the header, which is necessary for the processing of the packet. Source Address provides the origin IP address of the packet, allowing routers and devices on the network to identify the sender.

Therefore, recognizing the roles of these fields within the IP packet structure is essential for understanding the proper formation and function of IP packets in networking.