# /30 Subnet Mask (255.255.255.252) – Hosts, Range & FAQ

> The /30 subnet mask is 255.255.255.252. A /30 block holds 4 addresses with 2 usable hosts — ranges, wildcard, binary math and FAQs.

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# /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.

The /30 subnet mask is 255.255.255.252, equivalent to 0.0.0.3 as a wildcard. A /30 block contains 4 IP addresses (2 usable hosts).

## /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 10.0.0.4/30: 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?**

255.255.255.252, wildcard 0.0.0.3.

**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](https://itinsighthub.com/subnet-calculator/?q=198.51.100.0%2F30)

[Get the /30 wildcard mask →](https://itinsighthub.com/wildcard-mask-calculator/?q=%2F30)

### Related prefixes

[/24](https://itinsighthub.com/subnet/24/)[/26](https://itinsighthub.com/subnet/26/)[/28](https://itinsighthub.com/subnet/28/)[/29](https://itinsighthub.com/subnet/29/)[/31](https://itinsighthub.com/subnet/31/)

← /29 subnet mask
/31 subnet mask →

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