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

Question

Options

• A8, 6, 3
• B8, 8, 6
• C16, 8, 4
• D8, 6, 4
• E24, 16, 8

• A
  89% (17)
• B
  5% (1)
• C
  5% (1)

Explanation

Option A (8, 6, 3) correctly describes the original IPv4 Type of Service field as defined in RFC 791: the field is 8 bits wide, 6 bits are actively used (bits 0–5), and the 3 most significant bits define the IP Precedence value (ranging from 0 = routine to 7 = network control), while the remaining 2 bits (6–7) are reserved/unused.

Why distractors fail:

• B (8, 8, 6) is wrong on two counts - not all 8 bits are used (2 are reserved), and precedence occupies 3 bits, not 6.
• C (16, 8, 4) confuses the ToS field with a larger header field; the ToS is a single byte (8 bits), not 16.
• D (8, 6, 4) gets the total and used bits right but incorrectly assigns 4 bits to precedence instead of 3.
• E (24, 16, 8) bears no resemblance to any real IP header field structure.

Memory tip: Remember "8-6-3" as a descending triplet - the field is 1 byte (8), drops by 2 to the bits actually used (6), then drops by half for the precedence bits (3). You can also think of the 2 reserved bits as the "wasted" portion: 8 − 2 = 6 used, and the first 3 of those define who gets priority.

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