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IPv4 CIDR Subnet Range: Finding the First, Last, and Total Addresses for 205.16.37.39/28

IPv4 CIDR Subnet Range: Finding the First, Last, and Total Addresses for 205.16.37.39/28

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Nov 3, 2026

You are given an IPv4 CIDR block: 205.16.37.39/28. A /28 means the subnet mask is 255.255.255.240, i.e., the last 4 bits are host bits and the block size is 24=162^{4}=16 total IPv4 addresses in the range.
To find the first address and last address in the block, we first compute the network address (block start) by zeroing all host bits, then compute the broadcast address (block end) by setting all host bits to 1.

Key CIDR concepts you’ll use:

  • CIDR
  • Subnet mask
  • Network address
  • Broadcast address

Footnotes

  1. Subnet Cheat Sheet – 24 Subnet Mask, 30, 26, 27, 29, and other IP Address CIDR Network References - Includes /28 table showing # of IP addresses (16) and relation to CIDR. ↩

  2. How to find the first and last address in a block of IP addresses and understand subnetting 🌐 - DEV Community - Explains network (first) vs broadcast (last) using host bits 0/1 and AND with subnet mask. ↩

IPv4 Subnetting (CIDR /28) — Network, Broadcast, Usable Hosts

Step 1: Convert /28 to a subnet mask

For IPv4, a /28 prefix corresponds to subnet mask 255.255.255.240.

So the mask is:

  • 255.255.255.240 = 11111111.11111111.11111111.11110000 (binary interpretation)

Footnotes

  1. IPv4 CIDR Chart 2015 (RIPE) - CIDR prefix-to-subnet-mask mapping; includes /28 = 255.255.255.240. ↩

Compute first address (network) and last address (broadcast) for 205.16.37.39/28

  1. 1
    Step 1

    With /28, the increment is 16 in the last octet. So last-octet values group as: 0–15, 16–31, 32–47, 48–63, etc. (Because the block size is 16 addresses.)

    Footnotes

    1. Subnet Cheat Sheet – 24 Subnet Mask, 30, 26, 27, 29, and other IP Address CIDR Network References - Includes /28 table showing # of IP addresses (16) and relation to CIDR. ↩

  2. 2
    Step 2

    205.16.37.39 falls in the range 32–47 for the last octet, so the network (first address) is 205.16.37.32. This corresponds to zeroing the host bits.

    Footnotes

    1. How to find the first and last address in a block of IP addresses and understand subnetting 🌐 - DEV Community - Explains network (first) vs broadcast (last) using host bits 0/1 and AND with subnet mask. ↩

  3. 3
    Step 3

    The broadcast is the last value in that 32–47 range, so it is 205.16.37.47 (host bits all 1).

    Footnotes

    1. How to find the first and last address in a block of IP addresses and understand subnetting 🌐 - DEV Community - Explains network (first) vs broadcast (last) using host bits 0/1 and AND with subnet mask. ↩

  4. 4
    Step 4

    A /28 block contains 232−28=24=162^{32-28}=2^{4}=16 total IPv4 addresses.

    Footnotes

    1. Subnet Cheat Sheet – 24 Subnet Mask, 30, 26, 27, 29, and other IP Address CIDR Network References - Includes /28 table showing # of IP addresses (16) and relation to CIDR. ↩

Final answers

  • First address (network address): 205.16.37.32
  • Last address (broadcast address): 205.16.37.47
  • Total number of addresses in the block: 16

These follow directly from:

  • /28 subnet size is 16 addresses.
  • Network address is the block start; broadcast address is the block end.

Footnotes

  1. Subnet Cheat Sheet – 24 Subnet Mask, 30, 26, 27, 29, and other IP Address CIDR Network References - Includes /28 table showing # of IP addresses (16) and relation to CIDR. ↩

  2. How to find the first and last address in a block of IP addresses and understand subnetting 🌐 - DEV Community - Explains network (first) vs broadcast (last) using host bits 0/1 and AND with subnet mask. ↩

Address range summary for 205.16.37.39/28

First address is the network address; last address is the broadcast address.

Pro Tip: Don’t mix up “usable hosts” vs “block endpoints”

In many exercises, “first” and “last” for the block mean network and broadcast. Usable host IPs (if needed) are the addresses in between: network+1 through broadcast-1. For /28, that’s 14 usable hosts (but you only asked for first/last in the block).

Footnotes

  1. Subnet Cheat Sheet – 24 Subnet Mask, 30, 26, 27, 29, and other IP Address CIDR Network References - Includes /28 table showing # of IP addresses (16) and relation to CIDR. ↩

Quick FAQ

Knowledge Check

Question 1 of 4
Q1Single choice

For the CIDR block 205.16.37.39/28, what is the first address in the block?

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