diff --git a/ip/ip.d.ts b/ip/ip.d.ts
index 0542ae0785..1c00f1320e 100644
--- a/ip/ip.d.ts
+++ b/ip/ip.d.ts
@@ -14,6 +14,7 @@ interface SubnetInfo {
     subnetMaskLength: number;
     numHosts: number;
     length: number;
+    contains(ip: string): boolean;
 }
 
 declare module "ip" {
@@ -112,4 +113,4 @@ declare module "ip" {
      * @param cidr CIDR address.
      */
     export function cidrSubnet(cidr: string): SubnetInfo;
-}
\ No newline at end of file
+}