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Subnetting a Class B Address with a 6-Bit Subnet Number: Maximum Subnets, Hosts per Subnet, and Total Class B Networks

Subnetting a Class B Address with a 6-Bit Subnet Number: Maximum Subnets, Hosts per Subnet, and Total Class B Networks

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Nov 4, 2026

In classful IPv4 addressing, a Class B network uses a /16 network prefix, leaving 16 bits for host IDs within the class. When you subnet a Class B address using a 6-bit subnet number, the subnetting changes how many distinct networks exist and how many hosts each subnet can support.

Key formulas (from binary subnetting principles) are:

  • Maximum number of subnets: 262^{6}
  • Remaining host bits per subnet: 16−6=1016-6=10
  • Maximum hosts per subnet (usable): 210−22^{10}-2

This yields a maximum of 6464 subnets, with 10221022 usable hosts per subnet.

A further question asks: how many networks are allowed under Class B addresses in classful terms (i.e., without extra subnetting)? For classful Class B, the number of distinct network IDs is 216=655362^{16}=65536. 2

Footnotes

  1. Classful IP addressing - Class B - Explains Class B uses a /16 network prefix. ↩

  2. Classful network addressing (overview) - Provides the structure and counts for classful networks, including Class B network size. ↩

Subnetting basics: how subnet bits affect # of subnets and # of hosts

Learning goals

By the end of this section you can:

  1. Compute the maximum number of subnets given a borrowed subnet-bit length.
  2. Compute the maximum usable hosts per subnet accounting for network and broadcast addresses.
  3. Compute the total number of possible Class B network IDs under classful addressing.

Compute subnets and hosts for a Class B address with a 6-bit subnet number

  1. 1
    Step 1

    Class B uses a /16 network prefix, leaving 16 host bits. So total address bits = 32, network bits = 16, host bits = 16.

    Footnotes

    1. Classful IP addressing - Class B - Explains Class B uses a /16 network prefix. ↩

  2. 2
    Step 2

    A 6-bit subnet number means subnet bits = 6, borrowed from the original 16 host bits.

    Footnotes

    1. Subnetting (IPv4) - subnet masks and host counts - Covers borrowing bits, and the standard host-count rule 2h−22^{h}-2 for usable addresses. ↩

  3. 3
    Step 3

    With 6 subnet bits, maximum subnets = 26=642^{6}=64.

    Footnotes

    1. Subnetting (IPv4) - subnet masks and host counts - Covers borrowing bits, and the standard host-count rule 2h−22^{h}-2 for usable addresses. ↩

  4. 4
    Step 4

    Remaining host bits = 16−6=1016-6=10.

    Footnotes

    1. Subnetting (IPv4) - subnet masks and host counts - Covers borrowing bits, and the standard host-count rule 2h−22^{h}-2 for usable addresses. ↩

  5. 5
    Step 5

    Each subnet with 1010 host bits has 2102^{10} total addresses; subtract 2 for the subnet’s network and broadcast addresses → usable hosts = 210−2=10222^{10}-2=1022.

    Footnotes

    1. Subnetting (IPv4) - subnet masks and host counts - Covers borrowing bits, and the standard host-count rule 2h−22^{h}-2 for usable addresses. ↩

Rule of thumb for host counting

If you borrow ss bits for subnetting from a classful host field of size hh, then usable hosts per subnet are 2h−s−22^{h-s}-2 (assuming standard IPv4 with network & broadcast reserved).

IPv4 vs special cases

The −2-2 subtraction assumes subnet network and broadcast addresses are not usable for host assignment (standard classful/subnetting). Some edge cases (e.g., special OS configurations, or nonstandard interpretations) may differ, but the standard homework/CCNA-style answer uses 2k−22^{k}-2.

Final answers (direct)

  • Maximum number of subnets with a 6-bit subnet number:
    26=642^{6}=64
  • Maximum number of hosts in each subnet (usable host addresses):
    210−2=10222^{10}-2=1022
    (because Class B has 16 host bits total, and you borrow 6 bits, leaving 10 host bits per subnet). 2

Footnotes

  1. Classful IP addressing - Class B - Explains Class B uses a /16 network prefix. ↩

  2. Subnetting (IPv4) - subnet masks and host counts - Covers borrowing bits, and the standard host-count rule 2h−22^{h}-2 for usable addresses. ↩

Impact of borrowing 6 subnet bits from a Class B host field

Class B has 16 host bits; borrowing 6 creates subnets and reduces per-subnet host capacity.

Conceptual subnetting workflow

Identify classful host bits

1

Class B → /16 prefix → 16 host bits."

Borrow subnet bits

2

Subnet number length = 6 bits."

Compute number of subnets

3

Subnets = 262^{6}."

Compute usable hosts per subnet

4

Hosts = 210−22^{10}-2."

What is the number of networks allowed under Class B addresses?

Knowledge Check

Question 1 of 4
Q1Single choice

A Class B address has how many host bits in classful form?

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