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

@@ -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

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>