IPv4 TTL Field: Length, Range, and Loop Prevention

H12-811 By islever Editorial Last updated

IP Routing Basics Network Reference Models

Short answer

ABCD The 8-bit IPv4 TTL field ranges 0–255, decrements at each Layer 3 hop, and expires packets to break routing loops.

Question

Which of the following statements are true about the TTL field in an IPv4 header? (Choose all that apply.)
  1. The TTL field length is 8 bits. Correct Answer
  2. The TTL value decreases by 1 each time a packet passes through a Layer 3 device. Correct Answer
  3. When a routing loop occurs, the TTL value can be used to prevent infinite packet forwarding. Correct Answer
  4. The TTL value is in the range from 0 to 255. Correct Answer

Key Takeaways

The exam checks both the field's arithmetic (8 bits → 0–255) and its runtime behaviour (decrement per router hop, drop at zero), and the trap is dismissing the valid range 0–255 because value 0 is never sent as an initial TTL.

This H12-811 question tests the IPv4 TTL field: it is 8 bits wide, ranges from 0 to 255, is decremented by one at each Layer 3 hop, and stops packets circulating in routing loops — so all four statements A, B, C and D are correct (ABCD).

Common mistake: Choosing ABC and rejecting D, because a sender never sets TTL 0; the option states the field's representable range (2⁸ values, 0–255), not the values typically chosen at origination, so D is true.

Expert Analysis

The 8-bit field boundary The IPv4 header dedicates exactly one octet to Time To Live, so option A is correct and option D follows arithmetically: an unsigned 8-bit field can encode 2⁸ = 256 distinct values, i.e. 0 through 255. The trap here is to reject D because routers never originate a packet with TTL 0 (a packet carrying 0 is already undeliverable). The statement describes the field's representable range, not the set of values a sender typically chooses, so D stands.

Where TTL is enforced: the Layer 3 boundary TTL is decremented only by Layer 3 devices — routers — not by switches, hubs or repeaters operating at Layer 2. That is precisely what option B asserts, so B is correct. When the decrement drives TTL to 0, the router discards the packet and originates an ICMP "Time Exceeded" message back to the source. This is the mechanism behind option C: in a routing loop, packets circulate only until their TTL is exhausted, bounding the wasted bandwidth instead of forwarding forever. C is therefore correct too.

TTL versus IPv6 Hop Limit The concept tested here is often confused with IPv6's Hop Limit field, which occupies the same position and behaves identically but is named differently and defined in RFC 8200 rather than RFC 791. Nothing in the scenario involves IPv6, so the IPv4 TTL rules apply unchanged.

All four statements — A, B, C and D — are true, making ABCD the correct selection despite a suggested key of ABC.

Official Reference

https://www.rfc-editor.org/rfc/rfc791

This link opens the official vendor documentation in a new tab.

Exam Strategy

In multi-select header-field questions, translate each option into a bit-count or protocol-behaviour claim and verify it independently: 8 bits implies a 0–255 range, so A and D stand or fall together. Then apply the Layer 2 versus Layer 3 test — anything said to change at a switch is suspect, while TTL decrement and loop termination happen at routers. Whenever every option survives these two checks, select all of them rather than looking for a hidden distractor.

Frequently Asked Questions

Does TTL really have the value 0 as a valid range point?

The 8-bit field can represent 0 through 255, so 0 is in range. In practice a sender sets an initial TTL like 64, 128 or 255; once a router decrements TTL to 0 it discards the packet and returns ICMP Time Exceeded.

Why is TTL said to prevent infinite forwarding in a routing loop?

Each router hop subtracts 1, so a looping packet's TTL drains to zero after at most 255 hops and is dropped. This bounds loop traffic instead of letting packets circulate forever.

Is TTL the same as the IPv6 Hop Limit field?

They are functionally identical — decremented per Layer 3 hop and dropped at zero — but IPv6 names the field Hop Limit (RFC 8200), while RFC 791 defines it as TTL for IPv4.

More H12-811 FAQ →

Community Votes

A
34%
B
33%
C
33%

34% of anonymous learners picked answer A. Votes are pick records left by other test-takers — they are not the verified answer.

Discussion 2 contributions

Compiled from public community threads and reader submissions, lightly edited for clarity. Handles and vote tallies are shown as recorded, and comments reflect the views of individual contributors.

From public community threads 2

  1. Fmoura17 👍 1
    A, B, C, and D — All statements are true. Explanation A ✓ — The TTL field is 8 bits long in the IPv4 header. B ✓ — Each Layer 3 device (router) decrements the TTL by 1 when forwarding a packet. C ✓ — When TTL reaches 0, the router drops the packet and sends an ICMP "Time Exceeded" message, preventing infinite loops. D ✓ — Since TTL is 8 bits, its range is 0 to 255 (2⁸ = 256 possible values).
  2. Heidorn 👍 1
    All are true. The TTL header is indeed 8 bits

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