The IP ToS field consists of ________ bits, of which ________are used. The _________most significant (first) bits define precedence.

The original IP Type of Service (ToS) field, as defined in RFC 791, is an 8-bit field within the IPv4 header. Its purpose is to indicate the desired quality of service for a datagram. The structure is as follows: the three most significant bits (bits 0-2) are designated for IP Precedence. The next three bits (bits 3-5) are flags for Delay, Throughput, and Reliability (D, T, R). The final two bits (bits 6-7) were reserved for future use and were unused. Therefore, the field has 8 bits in total, of which 6 were actively used, and the first 3 bits define precedence.

B. This is incorrect. Only 6 of the 8 bits were defined for use (3 for precedence, 3 for DTR flags), and precedence used 3 bits, not 6.
C. This is incorrect. The IP ToS field is 8 bits long, not 16.
D. This is incorrect. IP Precedence is defined by the first 3 bits, not 4.
E. This is incorrect. The IP ToS field is 8 bits long, not 24.

1. Nokia Networks, "Nokia Quality of Service Student Guide" (Course Number SRC301). In Module 2, "QoS Models and Policy," the section on Layer 3 classification criteria details the structure of the original 8-bit ToS byte, specifying the 3-bit IP Precedence field and the 3 DTR bits, with the remaining 2 bits being unused.

2. Postel, J. (1981). RFC 791: Internet Protocol. Internet Engineering Task Force (IETF). Section 3.1, "Format," page 12, explicitly defines the Type of Service byte: "Bits 0-2: Precedence... Bits 3: D... 4: T... 5: R... Bits 6-7: Reserved for Future Use." This confirms the 8-bit total, 3-bit precedence, and 2 unused bits (meaning 6 were used).

3. Kurose, J. F., & Ross, K. W. (2017). Computer Networking: A Top-Down Approach (7th ed.). Pearson. Chapter 4, "The Network Layer: Data Plane," in the section discussing the IPv4 datagram format, illustrates the 8-bit Type of Service (ToS) field and its historical breakdown into IP Precedence and DTR bits.