From 01a20181b41851b188cd9b7b12a2e883b0ed4a46 Mon Sep 17 00:00:00 2001
From: Nick Chambers <uplime@spookyinternet.com>
Date: Sun, 25 Jul 2021 20:38:27 -0500
Subject: [PATCH] bash/cidr: ip routing calculator for convenience

---
 bash/cidr | 68 +++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 68 insertions(+)
 create mode 100755 bash/cidr

diff --git a/bash/cidr b/bash/cidr
new file mode 100755
index 0000000..66b6672
--- /dev/null
+++ b/bash/cidr
@@ -0,0 +1,68 @@
+#!/usr/bin/env bash
+
+bin() {
+  local str num=$1
+
+  while (( num > 0 )); do
+    str=$(( num % 2 ))$str
+    (( num /= 2 ))
+  done
+
+  while (( ${#str} < 8 )); do
+    str=0$str
+  done
+
+  printf %s "$str"
+}
+
+dec() {
+  local pow=$(( ${#1} - 1 )) num=0
+
+  for (( count = 0; count < ${#1}; ++count )); do
+    (( num += ${1:count:1} * (2 ** pow) ))
+    (( pow -= 1 ))
+  done
+
+  printf %d "$num"
+}
+
+2ip() {
+  local ip=( ) count
+
+  for count in {0..3}; do
+    ip[$count]=$(dec "${1:count * 8:8}")
+  done
+
+  printf %s.%s.%s.%s "${ip[@]}"
+}
+
+if (( ! $# )); then
+  printf 'usage: %s ip/cidr\n' "$0"
+  exit 1
+fi
+
+IFS=/ read -r ip size <<< "$1"
+IFS=. read -ra octets <<< "$ip"
+
+for (( count = 0; count < 4; ++count )); do
+  octets[$count]=$(bin "${octets[$count]}")
+done
+
+IFS= ip=${octets[*]} network=${ip:0:size} broadcast=${ip:0:size}
+
+while (( ${#network} < 32 )); do
+  network+=0
+  broadcast+=1
+done
+
+for (( count=0; count < size; ++count )); do
+  mask+=1
+done
+
+for (( count=32 - size; count < 32; ++count )); do
+  mask+=0
+done
+
+printf 'subnet mask: %s\n' "$(2ip "$mask")"
+printf 'range: %s - %s\n' "$(2ip "$network")" "$(2ip "$broadcast")"
+printf 'total assignable hosts: %d\n' "$(( (2 ** (32 - size)) - 2 ))"