After several sessions of scratching my chin thinking about a text based domain name and coming up with no good ideas, or finding out that the best ones where already taken, I decided to simplify the whole thing and just pick a number. If you can pick any number, why not picking a cool number…

What is a strobogrammatic prime number?

From Wikipedia

A strobogrammatic number is a number whose numeral is rotationally symmetric, so that it appears the same when rotated 180 degrees. In other words, the numeral looks the same right-side up and upside down (e.g., 69, 96, 1001). A strobogrammatic prime is a strobogrammatic number that is also a prime number, i.e., a number that is only divisible by one and itself.

Finding a strobogrammatic prime number

I put together a simple script to find the numbers

#!/bin/env python3
# Find strobogrammatic prime numbers

# Implementation of the AKS algorith
# Source https://stackoverflow.com/a/1801446
def isPrime(n):
    '''Returns True if n is a prime number'''
    if (n == 2) or (n == 3):
        return True
    if (n % 2 == 0) or (n % 3 == 0):
        return False

    i = 5
    w = 2
    while i * i <= n:
        if n % i == 0:
            return False
        i += w
        w = 6 - w
    return True

# Source https://github.com/kamyu104/LeetCode/blob/master/Python/strobogrammatic-number.py
def isStrobogrammatic(num):
    '''Returns True if n is a strobogrammatic string'''
    lookup = {'0':'0', '1':'1', '6':'9', '8':'8', '9':'6'}

    n = len(num)
    for i in range(round((n+1) / 2)):
        if num[n-1-i] not in lookup or \
           num[i] != lookup[num[n-1-i]]:
            return False
    return True

# I only want numbers of 5 and 6 digits
x = 10001
while x <= 999999:
    if isStrobogrammatic(str(x)):
      if isPrime(x):
        print("strobogrammatic prime: ",x)
$ time python3 ./strobogrammatic_prime_number_finder.py
strobogrammatic prime:  16091
strobogrammatic prime:  18181
strobogrammatic prime:  19861
strobogrammatic prime:  61819
strobogrammatic prime:  116911
strobogrammatic prime:  119611
strobogrammatic prime:  160091
strobogrammatic prime:  169691
strobogrammatic prime:  191161
strobogrammatic prime:  196961
strobogrammatic prime:  686989
strobogrammatic prime:  688889
python3 ./strobogrammatic_prime_number_finder.py  0.83s user 0.01s system 98% cpu 0.852 total

And the winner is… 191161.xyz

Because I want my dns to support DNSSEC (reasons…) I needed to pick a TLD which root zone has been signed. I choose some of the prime numbers and a couple cheap TLD, and after reviewing my options, I locked in with 191161.xyz for $5 a year.