From 040cbef9be9c158f968fc8c1679c196121413990 Mon Sep 17 00:00:00 2001
From: Your Name <you@example.com>
Date: Sat, 5 Jan 2019 16:09:41 -0600
Subject: [PATCH] Displays the correct number of available IP addresses for a
 subnet

create_subnet and describe_subnets displays the correct number of
available IP addresses. The module currently has '251' hardcoded.
This fix shows available IPs minus 5, which are reserved in each
subnet by AWS.
---
 moto/ec2/models.py            | 1 +
 moto/ec2/responses/subnets.py | 4 ++--
 2 files changed, 3 insertions(+), 2 deletions(-)

diff --git a/moto/ec2/models.py b/moto/ec2/models.py
index efbbeb6fe..dc12f8eba 100644
--- a/moto/ec2/models.py
+++ b/moto/ec2/models.py
@@ -2819,6 +2819,7 @@ class Subnet(TaggedEC2Resource):
         self.vpc_id = vpc_id
         self.cidr_block = cidr_block
         self.cidr = ipaddress.IPv4Network(six.text_type(self.cidr_block), strict=False)
+        self.available_ips = ipaddress.IPv4Network(cidr_block).num_addresses - 5
         self._availability_zone = availability_zone
         self.default_for_az = default_for_az
         self.map_public_ip_on_launch = map_public_ip_on_launch
diff --git a/moto/ec2/responses/subnets.py b/moto/ec2/responses/subnets.py
index c42583f23..e11984e52 100644
--- a/moto/ec2/responses/subnets.py
+++ b/moto/ec2/responses/subnets.py
@@ -53,7 +53,7 @@ CREATE_SUBNET_RESPONSE = """
     <state>pending</state>
     <vpcId>{{ subnet.vpc_id }}</vpcId>
     <cidrBlock>{{ subnet.cidr_block }}</cidrBlock>
-    <availableIpAddressCount>251</availableIpAddressCount>
+    <availableIpAddressCount>{{ subnet.available_ip_addresses }}</availableIpAddressCount>
     <availabilityZone>{{ subnet._availability_zone.name }}</availabilityZone>
     <availabilityZoneId>{{ subnet._availability_zone.zone_id }}</availabilityZoneId>
     <defaultForAz>{{ subnet.default_for_az }}</defaultForAz>
@@ -81,7 +81,7 @@ DESCRIBE_SUBNETS_RESPONSE = """
         <state>available</state>
         <vpcId>{{ subnet.vpc_id }}</vpcId>
         <cidrBlock>{{ subnet.cidr_block }}</cidrBlock>
-        <availableIpAddressCount>251</availableIpAddressCount>
+        <availableIpAddressCount>{{ subnet.available_ip_addresses }}</availableIpAddressCount>
         <availabilityZone>{{ subnet._availability_zone.name }}</availabilityZone>
         <availabilityZoneId>{{ subnet._availability_zone.zone_id }}</availabilityZoneId>
         <defaultForAz>{{ subnet.default_for_az }}</defaultForAz>