code: public MockSubnet createSubnet(
              final String cidrBlock, final String vpcId) {
  
          MockSubnet ret = new MockSubnet();
          ret.setCidrBlock(cidrBlock);
  
          ret.setSubnetId(
                  "subnet-" + UUID.randomUUID().toString().substring(0, SUBNET_ID_POSTFIX_LENGTH));
          ret.setVpcId(vpcId);
  
          allMockSubnets.put(ret.getSubnetId(), ret);
          return ret;
      }
docs: Create the mock Subnet.
  @param cidrBlock VPC cidr block.
  @param vpcId vpc Id for subnet.
  @return mock Subnet.