How many /29 networks fit
| Parent block | Relationship |
|---|---|
| /8 | 2,097,152 × /29 fit inside one /8 |
| /16 | 8,192 × /29 fit inside one /16 |
| /24 | 32 × /29 fit inside one /24 |
32 × /29 = one /24. A /29 holds 2 × /30 or 4 × /31.
Worked example: 198.51.100.83/29
Your ISP's email says: "Static block 198.51.100.83/29, gateway .86". What block is that really, and which addresses can you use?
- The mask is 255.255.255.248. The fourth octet (248) is the "interesting" one — the first that is not 255.
- Block size = 256 − 248 = 8. Networks start at multiples of 8 in the fourth octet: 0, 8, 16, …
- The fourth octet of 198.51.100.83 is 83. 83 ÷ 8 = 10 remainder 3, so the block starts at 10 × 8 = 80.
- Network = 198.51.100.80. Broadcast = 80 + 8 − 1 = 87, giving 198.51.100.87.
- Usable hosts: 198.51.100.81 to 198.51.100.86, which is 23 − 2 = 6 addresses.
Same thing in binary (network bits | host bits):
Address 11000110.00110011.01100100.01010|011 Mask 11111111.11111111.11111111.11111|000 Network 11000110.00110011.01100100.01010|000
Where /29 is used in real networks
-
"5 usable static IPs"
A /29 has 6 usable addresses. The ISP router takes one as your gateway, which leaves the famous 5 for you.
-
Smallest Google Cloud and Azure subnets
Google Cloud reserves 4 addresses per subnet and Azure reserves 5, so a /29 gives 4 usable in GCP and 3 in Azure.
-
Small office or branch segments
A management network for a few switches, or a small transit segment shared by two firewalls and a router.
Common /29 mistakes
- Taking the address in the email as the network address. 198.51.100.83 is in the block 198.51.100.80/29 (hosts .81 to .86, broadcast .87). Always work out the block from the mask.
- Configuring the firewall with mask 255.255.255.0 because "that's the usual one". The firewall then thinks hosts in other customers' blocks are local and never sends their traffic to the gateway.
/29 FAQ
How many usable IPs are in a /29?
6 on a normal network. With the ISP gateway taking one, you get 5. In Azure 3 and in Google Cloud 4, because of reserved addresses.
What is the subnet mask for /29?
255.255.255.248. The last octet is 11111000 in binary (248), leaving 3 host bits for 8 addresses. The wildcard is 0.0.0.7.
What are valid /29 starting addresses?
Any address whose last octet is a multiple of 8: .0, .8, .16, … .248.