{"id":11161,"date":"2026-01-01T14:19:30","date_gmt":"2026-01-01T19:19:30","guid":{"rendered":"https:\/\/stevepedwards.today\/DebianAdmin\/?p=11161"},"modified":"2026-01-01T14:19:44","modified_gmt":"2026-01-01T19:19:44","slug":"what-happens-past-30-in-subnetting","status":"publish","type":"post","link":"https:\/\/stevepedwards.today\/DebianAdmin\/what-happens-past-30-in-subnetting\/","title":{"rendered":"What Happens Past \/30 in Subnetting?"},"content":{"rendered":"<div class=\"pvc_clear\"><\/div>\n<p id=\"pvc_stats_11161\" class=\"pvc_stats all  \" data-element-id=\"11161\" style=\"\"><i class=\"pvc-stats-icon medium\" aria-hidden=\"true\"><svg aria-hidden=\"true\" focusable=\"false\" data-prefix=\"far\" data-icon=\"chart-bar\" role=\"img\" xmlns=\"http:\/\/www.w3.org\/2000\/svg\" viewBox=\"0 0 512 512\" class=\"svg-inline--fa fa-chart-bar fa-w-16 fa-2x\"><path fill=\"currentColor\" d=\"M396.8 352h22.4c6.4 0 12.8-6.4 12.8-12.8V108.8c0-6.4-6.4-12.8-12.8-12.8h-22.4c-6.4 0-12.8 6.4-12.8 12.8v230.4c0 6.4 6.4 12.8 12.8 12.8zm-192 0h22.4c6.4 0 12.8-6.4 12.8-12.8V140.8c0-6.4-6.4-12.8-12.8-12.8h-22.4c-6.4 0-12.8 6.4-12.8 12.8v198.4c0 6.4 6.4 12.8 12.8 12.8zm96 0h22.4c6.4 0 12.8-6.4 12.8-12.8V204.8c0-6.4-6.4-12.8-12.8-12.8h-22.4c-6.4 0-12.8 6.4-12.8 12.8v134.4c0 6.4 6.4 12.8 12.8 12.8zM496 400H48V80c0-8.84-7.16-16-16-16H16C7.16 64 0 71.16 0 80v336c0 17.67 14.33 32 32 32h464c8.84 0 16-7.16 16-16v-16c0-8.84-7.16-16-16-16zm-387.2-48h22.4c6.4 0 12.8-6.4 12.8-12.8v-70.4c0-6.4-6.4-12.8-12.8-12.8h-22.4c-6.4 0-12.8 6.4-12.8 12.8v70.4c0 6.4 6.4 12.8 12.8 12.8z\" class=\"\"><\/path><\/svg><\/i> <img loading=\"lazy\" decoding=\"async\" width=\"16\" height=\"16\" alt=\"Loading\" src=\"https:\/\/stevepedwards.today\/DebianAdmin\/wp-content\/plugins\/page-views-count\/ajax-loader-2x.gif\" border=0 \/><\/p>\n<div class=\"pvc_clear\"><\/div>\n<p>What Happens Past \/30 in Subnetting?<\/p>\n<p>While a <strong>\/30<\/strong> 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 <strong>\/31<\/strong> and <strong>\/32<\/strong> for specific reasons.<\/p>\n<p>Here is why those masks exist and where you\u2019ll see them in a configuration:<\/p>\n<p><strong>1. The \/31 Mask: The \"Efficient\" Point-to-Point<\/strong><\/p>\n<p>You are exactly right: on a point-to-point link, there are only two devices. If Router A sends a packet, it can <em>only<\/em> go to Router B. There is no need for a broadcast address because there\u2019s no one else to \"shout\" to.<\/p>\n<ul>\n<li><strong>The Problem with \/30:<\/strong> It uses 4 IP addresses (e.g., .0 for Network, .1 and .2 for Hosts, and .3 for Broadcast). That\u2019s 50% waste.<\/li>\n<li><strong>The Solution (\/31):<\/strong> Defined in <strong>RFC 3021<\/strong>, 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.<\/li>\n<li><strong>Where you'll see it:<\/strong> Router-to-router links in ISP backbones or large data centers where saving IPv4 space is critical.<\/li>\n<\/ul>\n<p><strong>2. The \/32 Mask: The \"Single Host\"<\/strong><\/p>\n<p>A <strong>\/32<\/strong> mask means there are zero bits left for hosts, which mathematically defines a <strong>single IP address<\/strong>. You likely saw this in your router's WAN config for one of two reasons:<\/p>\n<ul>\n<li><strong>Loopback Interfaces:<\/strong> 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.<\/li>\n<li><strong>PPP \/ PPPoE Connections:<\/strong> 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.<\/li>\n<\/ul>\n<p><strong>Summary Comparison<\/strong><\/p>\n<table>\n<thead>\n<tr>\n<th><strong>Mask<\/strong><\/th>\n<th><strong>Total IPs<\/strong><\/th>\n<th><strong>Usable IPs<\/strong><\/th>\n<th><strong>Common Use Case<\/strong><\/th>\n<\/tr>\n<\/thead>\n<tbody>\n<tr>\n<td><strong>\/30<\/strong><\/td>\n<td>4<\/td>\n<td>2<\/td>\n<td>Legacy Point-to-Point (Standard \"classroom\" subnetting)<\/td>\n<\/tr>\n<tr>\n<td><strong>\/31<\/strong><\/td>\n<td>2<\/td>\n<td>2<\/td>\n<td>Modern Point-to-Point (ISP Backbones)<\/td>\n<\/tr>\n<tr>\n<td><strong>\/32<\/strong><\/td>\n<td>1<\/td>\n<td>1<\/td>\n<td>Loopback interfaces, VPN endpoints, or ISP WAN IPs (PPPoE)<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<p>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.<\/p>\n","protected":false},"excerpt":{"rendered":"<div class=\"pvc_clear\"><\/div>\n<p id=\"pvc_stats_11161\" class=\"pvc_stats all  \" data-element-id=\"11161\" style=\"\"><i class=\"pvc-stats-icon medium\" aria-hidden=\"true\"><svg aria-hidden=\"true\" focusable=\"false\" data-prefix=\"far\" data-icon=\"chart-bar\" role=\"img\" xmlns=\"http:\/\/www.w3.org\/2000\/svg\" viewBox=\"0 0 512 512\" class=\"svg-inline--fa fa-chart-bar fa-w-16 fa-2x\"><path fill=\"currentColor\" d=\"M396.8 352h22.4c6.4 0 12.8-6.4 12.8-12.8V108.8c0-6.4-6.4-12.8-12.8-12.8h-22.4c-6.4 0-12.8 6.4-12.8 12.8v230.4c0 6.4 6.4 12.8 12.8 12.8zm-192 0h22.4c6.4 0 12.8-6.4 12.8-12.8V140.8c0-6.4-6.4-12.8-12.8-12.8h-22.4c-6.4 0-12.8 6.4-12.8 12.8v198.4c0 6.4 6.4 12.8 12.8 12.8zm96 0h22.4c6.4 0 12.8-6.4 12.8-12.8V204.8c0-6.4-6.4-12.8-12.8-12.8h-22.4c-6.4 0-12.8 6.4-12.8 12.8v134.4c0 6.4 6.4 12.8 12.8 12.8zM496 400H48V80c0-8.84-7.16-16-16-16H16C7.16 64 0 71.16 0 80v336c0 17.67 14.33 32 32 32h464c8.84 0 16-7.16 16-16v-16c0-8.84-7.16-16-16-16zm-387.2-48h22.4c6.4 0 12.8-6.4 12.8-12.8v-70.4c0-6.4-6.4-12.8-12.8-12.8h-22.4c-6.4 0-12.8 6.4-12.8 12.8v70.4c0 6.4 6.4 12.8 12.8 12.8z\" class=\"\"><\/path><\/svg><\/i> <img loading=\"lazy\" decoding=\"async\" width=\"16\" height=\"16\" alt=\"Loading\" src=\"https:\/\/stevepedwards.today\/DebianAdmin\/wp-content\/plugins\/page-views-count\/ajax-loader-2x.gif\" border=0 \/><\/p>\n<div class=\"pvc_clear\"><\/div>\n<p>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\u2019ll see them in a configuration: 1. <a href=\"https:\/\/stevepedwards.today\/DebianAdmin\/what-happens-past-30-in-subnetting\/\" class=\"more-link\">...<span class=\"screen-reader-text\">\u00a0 What Happens Past \/30 in Subnetting?<\/span><\/a><\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[4],"tags":[],"class_list":["post-11161","post","type-post","status-publish","format-standard","hentry","category-post"],"a3_pvc":{"activated":true,"total_views":3,"today_views":0},"_links":{"self":[{"href":"https:\/\/stevepedwards.today\/DebianAdmin\/wp-json\/wp\/v2\/posts\/11161","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/stevepedwards.today\/DebianAdmin\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/stevepedwards.today\/DebianAdmin\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/stevepedwards.today\/DebianAdmin\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/stevepedwards.today\/DebianAdmin\/wp-json\/wp\/v2\/comments?post=11161"}],"version-history":[{"count":1,"href":"https:\/\/stevepedwards.today\/DebianAdmin\/wp-json\/wp\/v2\/posts\/11161\/revisions"}],"predecessor-version":[{"id":11162,"href":"https:\/\/stevepedwards.today\/DebianAdmin\/wp-json\/wp\/v2\/posts\/11161\/revisions\/11162"}],"wp:attachment":[{"href":"https:\/\/stevepedwards.today\/DebianAdmin\/wp-json\/wp\/v2\/media?parent=11161"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/stevepedwards.today\/DebianAdmin\/wp-json\/wp\/v2\/categories?post=11161"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/stevepedwards.today\/DebianAdmin\/wp-json\/wp\/v2\/tags?post=11161"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}