What Happens Past /30 in Subnetting?

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What Happens Past /30 in Subnetting?

While a /30 is the "classic" way to teach subnetting (because it follows the rule of needing a Network ID and a Broadcast address), modern networking frequently uses /31 and /32 for specific reasons.

Here is why those masks exist and where you’ll see them in a configuration:

1. The /31 Mask: The "Efficient" Point-to-Point

You are exactly right: on a point-to-point link, there are only two devices. If Router A sends a packet, it can only go to Router B. There is no need for a broadcast address because there’s no one else to "shout" to.

  • The Problem with /30: It uses 4 IP addresses (e.g., .0 for Network, .1 and .2 for Hosts, and .3 for Broadcast). That’s 50% waste.
  • The Solution (/31): Defined in RFC 3021, this allows a subnet with only two addresses (e.g., 10.0.0.0 and 10.0.0.1). Both are used as host IPs.
  • Where you'll see it: Router-to-router links in ISP backbones or large data centers where saving IPv4 space is critical.

2. The /32 Mask: The "Single Host"

A /32 mask means there are zero bits left for hosts, which mathematically defines a single IP address. You likely saw this in your router's WAN config for one of two reasons:

  • Loopback Interfaces: Routers often have a "virtual" interface called a Loopback. Since this interface exists only inside the router's software and isn't a physical wire with other neighbors, it only needs one IP. Network engineers use /32 for these so they don't waste any extra addresses.
  • PPP / PPPoE Connections: This is likely what you saw. Many home internet connections use PPPoE (Point-to-Point Protocol over Ethernet). In a PPP session, the link is a "tunnel." Your ISP assigns your router a single IP with a /32 mask because your router doesn't need to know about a local subnet; it just needs to know that any traffic not meant for your home network should be shoved down that tunnel to the ISP.

Summary Comparison

Mask Total IPs Usable IPs Common Use Case
/30 4 2 Legacy Point-to-Point (Standard "classroom" subnetting)
/31 2 2 Modern Point-to-Point (ISP Backbones)
/32 1 1 Loopback interfaces, VPN endpoints, or ISP WAN IPs (PPPoE)

Essentially, as you get further into real-world networking, the "rules" of needing a Network and Broadcast address start to disappear in favor of efficiency.