is-rfc1918.clj

(ns your-namespace
  (:use [clojure.string :only [split]]))

; authored by ordnungswidrig

(defn is-rfc1918?
  "Returns true if a string is a dotted quad in the IP ranges of 10.0.0.0/8,
   192.168.0.0/16 or 172.16.0.0/22, else returns nil)"
  [^String ip]
  (when-let [[a b c d :as qs]
             (map #(Short/valueOf %) (clojure.string/split ip #"\."))]
    (and (= 4 (count qs)) (every? #(<= 0 % 255) qs)
         (or (= 10 a)
             (= [192 168] [a b])
             (and (= 172 a) (<= 16 b 31))))))

(prn (is-rfc1918? "10.123.45.6"))

Thank you! I had forgotten about short / Short(s), too ; )