This MPLS label decoder splits 32-bit label stack entries from a packet capture into their fields and names the special-purpose labels, and builds entries from fields for lab work. It runs entirely in your browser: nothing you type is sent to our server.
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Table of Contents
What the MPLS label decoder reads: the 32-bit entry
| Bits | Field |
|---|---|
| 20 | Label (0-1,048,575) |
| 3 | Traffic Class (formerly EXP), for QoS |
| 1 | S: bottom of stack (1 on the last entry) |
| 8 | TTL |
Special-purpose labels
| Label | Meaning |
|---|---|
| 0 | IPv4 Explicit NULL |
| 1 | Router Alert |
| 2 | IPv6 Explicit NULL |
| 3 | Implicit NULL: signals penultimate hop popping, never sent |
| 7 | Entropy Label Indicator (RFC 6790) |
| 13 | GAL (RFC 5586) |
| 14 | OAM Alert (RFC 3429) |
| 15 | Extension Label (RFC 7274) |
Each label adds 4 bytes, so a two-label stack needs a core MTU of at least 1508 to carry 1500-byte IP packets. See the MTU and MSS calculator for tunnels.
Decoding an MPLS label stack entry: worked example
Each label stack entry is 32 bits: a 20-bit label, a 3-bit traffic class (TC), a 1-bit bottom-of-stack flag (S) and an 8-bit TTL. Take the entry 0001F140 from a capture:
0001F140 = 0000 0000 0000 0001 1111 | 000 | 1 | 0100 0000
label = 31 TC = 0 S = 1 (last entry) TTL = 6400003140 decodes to label 3, which is a special case: the implicit-null label is only signalled, never sent on the wire.
Common mistakes:
- Calling the TC bits “EXP”: they are the same three bits, renamed traffic class by RFC 5462.
- Mixing up the reserved labels: 0 is IPv4 explicit null, 2 is IPv6 explicit null, 3 is implicit null (penultimate hop popping); labels 0 to 15 are reserved.
- Reading S on the wrong entry: only the last entry of the stack has S set to 1.
Related tools and guides: BGP community decoder, IPv4 header checksum calculator, number base converter, OSPF cost calculator.
How we checked it
We tested the decoder on 11-12 October 2026 with a two-label stack (labels 1000 and 31), a single Implicit NULL entry and the encoder round trip, and checked the special-purpose label names against RFC 7274.
Reference: the MPLS label stack encoding is defined in RFC 3032.
Frequently asked questions
How long is an MPLS label?
The label value is 20 bits; the whole stack entry with TC, S and TTL is 32 bits (4 bytes).
What is label 3 in MPLS?
Implicit NULL. A router advertises it to ask the previous hop to pop the label (penultimate hop popping), so it never appears in packets.
What does the S bit mean?
Bottom of stack: it is 1 on the last label and 0 on all others.