Everything to Know About /29 Subnet Mask and Its 6 Usable IP Addresses

A /29 subnet mask, commonly written in its dotted-decimal form as 255.255.255.248, represents a critical building block in modern IPv4 networking. Whether you are a network administrator configuring a small office branch, a developer setting up cloud infrastructure, or a student preparing for the CCNA exam, understanding the math and the practical application of this specific CIDR (Classless Inter-Domain Routing) notation is essential.

For those looking for a quick reference, here are the core specifications of a /29 network:

Technical Breakdown of the /29 Subnet Mask

To truly master subnetting, one must look beyond the decimal numbers and understand the binary structure of an IPv4 address. An IPv4 address consists of 32 bits divided into four octets. The "/29" in CIDR notation indicates that the first 29 bits of the 32-bit address are reserved for the network prefix, leaving exactly 3 bits for host identification.

The Binary Logic

When we write out the /29 mask in binary, it looks like this: 11111111.11111111.11111111.11111000

The first three octets are completely filled with ones, which equals 255.255.255 in decimal. The fourth octet, 11111000, is where the subnetting magic happens. In binary-to-decimal conversion, the positions represent powers of two:

• 128 (1st bit)
• 64 (2nd bit)
• 32 (3rd bit)
• 16 (4th bit)
• 8 (5th bit)
• 4 (6th bit) - Off
• 2 (7th bit) - Off
• 1 (8th bit) - Off

Summing the "On" bits: $128 + 64 + 32 + 16 + 8 = 248$. This explains why the decimal representation is 255.255.255.248.

The Wildcard Mask: 0.0.0.7

In some routing protocols like OSPF or when configuring Access Control Lists (ACLs) on Cisco hardware, you may need the "wildcard mask." This is effectively the inverse of the subnet mask. By subtracting each octet of the subnet mask from 255, we get: $255 - 255 = 0$ $255 - 255 = 0$ $255 - 255 = 0$ $255 - 248 = 7$ Result: 0.0.0.7. This mask tells the router to ignore the first 29 bits and match only the last 3 bits.

Calculating IPs in a /29 Subnet: The Math Behind the 6 Usable Hosts

A common question is: "If /29 provides 8 total IP addresses, why can I only use 6?"

The answer lies in the fundamental rules of IPv4 networking. Every subnet requires two reserved addresses that cannot be assigned to hardware devices like servers, PCs, or printers.

1. The Network Address: This is the first address in the block (all host bits are 0). It is used to identify the sub-network itself within a routing table.
2. The Broadcast Address: This is the last address in the block (all host bits are 1). It is used to send data packets to every device on that specific subnet simultaneously.

The formula for calculating usable hosts is $2^h - 2$, where $h$ is the number of host bits. For a /29 network:

• Total Bits (32) - Network Bits (29) = 3 Host Bits.
• $2^3 = 8$ total addresses.
• $8 - 2 = 6$ usable host addresses.

Why 6 IPs Matter

In a professional setting, a /29 block is often the "Goldilocks" size for small public IP assignments. For example, if an ISP provides you with a /29 public block, your allocation might look like this:

• IP 1: The ISP's Gateway (your router's WAN interface).
• IP 2: Your primary firewall.
• IP 3: A public-facing Web Server.
• IP 4: An Email/SMTP Server.
• IP 5: A VPN Gateway.
• IP 6: A spare for future growth or a staging server.

Step-by-Step Example: Subnetting a Class C Range

Imagine you have a standard Class C network: 192.168.1.0/24. A /24 provides 256 addresses (254 usable). If you want to divide this into smaller segments using /29 masks, you can create exactly 32 subnets ($256 / 8 = 32$).

Let's look at the first few segments of this breakdown:

Subnet #1

• Network ID: 192.168.1.0
• Usable Range: 192.168.1.1 to 192.168.1.6
• Broadcast Address: 192.168.1.7

Subnet #2

• Network ID: 192.168.1.8
• Usable Range: 192.168.1.9 to 192.168.1.14
• Broadcast Address: 192.168.1.15

Subnet #3

• Network ID: 192.168.1.16
• Usable Range: 192.168.1.17 to 192.168.1.22
• Broadcast Address: 192.168.1.23

This pattern continues in increments of 8. If you accidentally try to assign 192.168.1.8 to a computer, your operating system will likely throw an error saying "Invalid IP address" or "This is a network address." Similarly, assigning 192.168.1.15 will fail because it is reserved for broadcasts.

Full Reference Table: All 32 Subnets in a /24 Block

For network engineers designing a Variable Length Subnet Mask (VLSM) scheme, having a quick look-up table is invaluable. Below is the full list of network boundaries for /29 subnets within a .0 to .255 range.

Real-World Use Cases: When to Deploy a /29 Subnet

In our experience managing enterprise networks, the /29 mask isn't just a theoretical exercise; it's a strategic tool for efficient IP management.

1. Small Business WAN Blocks

When you order a "static IP" package from a commercial ISP (like Comcast Business, AT&T, or BT), they rarely give you a single IP. Instead, they typically route a /29 block to your modem. Why? Because it provides enough room for the ISP's gateway and several of your own devices. If they gave you a /30 (2 usable IPs), you would only have one IP for your router and one for the ISP—leaving zero room for a secondary firewall or a DMZ server.

2. DMZ (Demilitarized Zone) Segmentation

Security-conscious organizations often isolate public-facing servers from the internal LAN. A /29 is perfect for a small DMZ containing a web server, an application proxy, and a database link. It provides enough addresses to maintain separation without wasting hundreds of IPs from a /24 block.

3. Management Networks

In a data center, you might use a /29 for an "Out-of-Band" management network for a single rack of equipment. This segment could connect the IPMI/iDRAC interfaces of 5 servers and 1 top-of-rack switch, keeping management traffic isolated from production data.

4. Site-to-Site VPN Hubs

In Hub-and-Spoke VPN topologies, the "Hub" might assign small /29 virtual adapters to different departments or branch offices. This ensures that even if one branch is compromised, the broadcast domain is limited to just 6 other potential targets rather than the entire corporate network.

Configuration Guide: Setting up /29 on Professional Hardware

Configuring a /29 subnet requires precision. If you enter the mask incorrectly (e.g., using 255.255.255.0 instead of 248), your device will expect a much larger network and fail to communicate with the gateway.

Cisco IOS Configuration

To assign a /29 IP to a GigabitEthernet interface: