/30 Subnet Mask ReferenceIPv4
Two hosts, two taxes: a /30 spends four addresses to connect exactly two devices, paying the network-and-broadcast toll on every link. It carried WAN circuits for decades, and it is precisely the waste RFC 3021 was written to fix.
/30 facts at a glance
- Subnet mask
- 255.255.255.252
- Wildcard mask
- 0.0.0.3
- Binary mask
- 11111111.11111111.11111111.11111100
- Total addresses
- 4
- Usable hosts
- 2 (excludes network & broadcast)
- Block size
- 4 addresses (22)
- Typical role
- Subnets, VLANs and WAN links
Example /30 networks
Three consecutive /30 blocks, starting at the /30 boundary that contains 198.51.100.0:
| Network | Usable range | Usable hosts |
|---|---|---|
| 198.51.100.0/30 | 198.51.100.1 – 198.51.100.2 | 2 |
| 198.51.100.4/30 | 198.51.100.5 – 198.51.100.6 | 2 |
| 198.51.100.8/30 | 198.51.100.9 – 198.51.100.10 | 2 |
How to calculate a /30 mask
A /30 mask sets the first 30 bits of a 32-bit number to 1 and leaves the remaining 2 bits as 0. In binary that reads 11111111.11111111.11111111.11111100, which converts to dotted decimal 255.255.255.252. Each block covers a contiguous run of 22 = 4 addresses, and every block starts at a multiple of that size. The wildcard simply flips every bit, giving 0.0.0.3.
Where /30 is used
- Legacy serial and Ethernet point-to-point circuits between routers.
- Older platforms whose interface software predates /31 support.
- Exam scenarios and audits that still assume one /30 per WAN link.
Anatomy of <code>10.0.0.4/30</code>: network 10.0.0.4, hosts 10.0.0.5 and 10.0.0.6, broadcast 10.0.0.7. Half the block is overhead.
Across thousands of WAN circuits, that fifty percent tax once consumed entire /24s worth of scarce address space, which is why RFC 3021 exists.
Frequently asked questions
Why does a /30 only have 2 usable hosts?
Four addresses exist, but the first is the network and the last is the broadcast, leaving exactly two for the two ends of the point-to-point link.
What is the subnet mask for a /30?
<code>255.255.255.252</code>, wildcard <code>0.0.0.3</code>.
Are /30s still used today?
Yes, mostly on legacy equipment that lacks RFC 3021 support. New point-to-point designs prefer /31s, which serve both endpoints without any waste.
How many /30s are in a /24?
Sixty-four, each consuming four addresses.
Open this block in a calculator
Open the /30 block in the subnet calculator