Inital commit

Dockerfile

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+FROM scratch
+COPY subnetcalc /subnetcalc
+ENTRYPOINT ["/subnetcalc"]

LICENSE

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+Copyright (C) 2020 Jack-Benny Persson <jack-benny@cyberinfo.se>
+
+This program is free software; you can redistribute it and/or modify
+it under the terms of the GNU General Public License as published by
+the Free Software Foundation; either version 2 of the License, or
+(at your option) any later version.
+
+This program is distributed in the hope that it will be useful,
+but WITHOUT ANY WARRANTY; without even the implied warranty of
+MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+GNU General Public License for more details.
+
+You should have received a copy of the GNU General Public License
+along with this program; if not, write to the Free Software
+Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
+

Makefile

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+subnetcalc: subnetcalc.c
+	gcc -Wall -pedantic subnetcalc.c -lm -static -o subnetcalc
+	docker build . -t subnetcalc
+
+clean: subnetcalc
+	rm subnetcalc
+	docker image rm subnetcalc

README.md

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+# subnetcalc #
+A simple subnet calculator written in C, statically compiled, and
+then Dockerized. The *Dockerized* version was created as a demonstration
+on how to create really small Docker images from scratch.
+
+![Docker Image Size (latest by date)](https://img.shields.io/docker/image-size/jackbenny/subnetcalc?sort=date)
+![Docker Pulls](https://img.shields.io/docker/pulls/jackbenny/subnetcalc)
+![Docker Stars](https://img.shields.io/docker/stars/jackbenny/subnetcalc)
+![Docker Cloud Automated build](https://img.shields.io/docker/cloud/automated/jackbenny/subnetcalc)
+![Docker Cloud Build Status](https://img.shields.io/docker/cloud/build/jackbenny/subnetcalc)
+
+## Manual build 
+If you would like to build it yourself, instead of running the
+pre-compiled image from Docker Hub, simple type `make`. This will
+create a image called *subnetcalc*. If you get a *"is up to date"* message,
+instead run `make --always-make`.
+
+## Usage
+Run it (remove jackbenny/ if you would like to run it from your local build)
+and enter the subnet mask in slash notation -- but without the slash. 
+
+For example enter 24 for a 24-bit subnet mask and the
+program will output the total number of addresses in the range aswell
+as the total number of usable addresses for hosts.
+
+    $> docker run -it --rm jackbenny/subnetcalc
+    Enter netmask in slash-notation without the slash: 24
+    Netmask bit: 24
+
+    256 total addresses
+    254 usable addresses for hosts
+

subnetcalc.c

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+/* A very simple subnet calculator in C, written by
+ * Jack-Benny Persson (jack-benny@cyberinfo.se).
+ * Released under GNU GPLv2.
+*/
+
+#include <stdio.h>
+#include <math.h>
+
+int main(void)
+{
+    int net;
+    double hosts;
+    double addr;
+    printf("Enter netmask in slash-notation without the slash: ");
+    scanf("%2d", &net);
+
+    if (net >= 0 && net <= 32) // Sanity check the user input
+    {
+        printf("Netmask bit: %d\n\n", net);	
+        addr = pow(2, 32-net); // Calculate number of addresses
+        printf("%.0lf total addresses\n", addr);
+
+        hosts = addr-2; 
+        if (hosts < 0)   // Check if number of usable hosts is a negative value
+        {
+            hosts = 0;   // Set usable hosts to zero if it was negative
+        }   
+        printf("%.0lf usable addresses for hosts\n", hosts);
+        return 0;
+    }
+    else  // If the user entered anything else than 0-32
+    {
+        printf("Only values between 0-32 are accepted\n");
+        return 1;
+    }
+
+    return 1;
+}