How many bits are used in an MPLS label to provide over 1 million unique labels?

• A. 16 bits
• B. 20 bits
• C. 24 bits
• D. 8 bits

Answer: (B)

MPLS uses a fixed-length 20-bit label field inside each label stack entry. With 20 bits, you can represent 2^20 distinct values, which equals 1,048,576—more than one million. That’s why this size is chosen: it provides far more than enough unique labels for large networks while keeping the header compact. The MPLS label stack entry actually packs 20 bits for the label value along with other fields (3 bits for EXP, 1 bit for Bottom of Stack, and 8 bits for TTL) into a 32-bit header. The other options correspond to smaller or larger bit widths, but the standard defines 20 bits for the label field, giving what’s needed to exceed a million unique labels.