/0 vs /1 — Subnet Comparison
A /0 subnet is 2× larger than a /1. Every additional bit in the prefix halves the address space — the 1-bit difference between these two means /0 has 21 = 2 times as many addresses.
The entire IPv4 internet
Typical Uses
- →Default route / route of last resort
- →Firewall "allow all" or "deny all" rule
- →BGP full routing table advertisement
Key Differences
How 2 /1 Subnets Divide a /0
Example using 10.0.0.0/0 as the parent block.
| # | CIDR | Network | First Usable | Last Usable | Broadcast | Hosts |
|---|---|---|---|---|---|---|
| 1 | 0.0.0.0/1 | 0.0.0.0 | 0.0.0.1 | 127.255.255.254 | 127.255.255.255 | 2,147,483,646 |
| 2 | 128.0.0.0/1 | 128.0.0.0 | 128.0.0.1 | 255.255.255.254 | 255.255.255.255 | 2,147,483,646 |
FAQ
What is the difference between /0 and /1?
A /0 has 4,294,967,294 usable hosts
and a /1 has 2,147,483,646.
The subnet masks differ: /0 uses 0.0.0.0
while /1 uses 128.0.0.0.
Every additional bit in the prefix halves the number of addresses — so the 1-bit gap means
/0 is exactly 2× larger.
How many /1 subnets fit in a /0?
Exactly 2 /1 subnets fit perfectly inside one /0 with no wasted space. To split a /0 into /1s, just increment the last 1 bit of the network address for each new subnet.
Which should I choose?
Choose based on how many hosts you need. Use the hosts → prefix calculator on the homepage to find the right size for your requirements.