Modifying opts and model structs with stricter types
diff --git a/openstack/networking/v2/subnets/requests.go b/openstack/networking/v2/subnets/requests.go
index f592739..ff36765 100644
--- a/openstack/networking/v2/subnets/requests.go
+++ b/openstack/networking/v2/subnets/requests.go
@@ -89,12 +89,20 @@
 	return &s, nil
 }
 
+// maybeString returns nil for empty strings and nil for empty.
+func maybeString(original string) *string {
+	if original != "" {
+		return &original
+	}
+	return nil
+}
+
 const (
 	IPv4 = 4
 	IPv6 = 6
 )
 
-type SubnetOpts struct {
+type CreateOpts struct {
 	// Required
 	NetworkID string
 	CIDR      string
@@ -104,19 +112,12 @@
 	AllocationPools []AllocationPool
 	GatewayIP       string
 	IPVersion       int
-	ID              string
 	EnableDHCP      *bool
+	DNSNameservers  []string
+	HostRoutes      []interface{}
 }
 
-// maybeString returns nil for empty strings and nil for empty.
-func maybeString(original string) *string {
-	if original != "" {
-		return &original
-	}
-	return nil
-}
-
-func Create(c *gophercloud.ServiceClient, opts SubnetOpts) (*Subnet, error) {
+func Create(c *gophercloud.ServiceClient, opts CreateOpts) (*Subnet, error) {
 	// Validate required options
 	if opts.NetworkID == "" {
 		return nil, ErrNetworkIDRequired
@@ -136,8 +137,9 @@
 		AllocationPools []AllocationPool `json:"allocation_pools,omitempty"`
 		GatewayIP       *string          `json:"gateway_ip,omitempty"`
 		IPVersion       int              `json:"ip_version,omitempty"`
-		ID              *string          `json:"id,omitempty"`
 		EnableDHCP      *bool            `json:"enable_dhcp,omitempty"`
+		DNSNameservers  []string         `json:"dns_nameservers,omitempty"`
+		HostRoutes      []interface{}    `json:"host_routes,omitempty"`
 	}
 	type request struct {
 		Subnet subnet `json:"subnet"`
@@ -149,17 +151,21 @@
 		Name:       maybeString(opts.Name),
 		TenantID:   maybeString(opts.TenantID),
 		GatewayIP:  maybeString(opts.GatewayIP),
-		ID:         maybeString(opts.ID),
 		EnableDHCP: opts.EnableDHCP,
 	}}
 
 	if opts.IPVersion != 0 {
 		reqBody.Subnet.IPVersion = opts.IPVersion
 	}
-
 	if len(opts.AllocationPools) != 0 {
 		reqBody.Subnet.AllocationPools = opts.AllocationPools
 	}
+	if len(opts.DNSNameservers) != 0 {
+		reqBody.Subnet.DNSNameservers = opts.DNSNameservers
+	}
+	if len(opts.HostRoutes) != 0 {
+		reqBody.Subnet.HostRoutes = opts.HostRoutes
+	}
 
 	type response struct {
 		Subnet *Subnet `json:"subnet"`
@@ -179,38 +185,38 @@
 	return res.Subnet, nil
 }
 
-func Update(c *gophercloud.ServiceClient, id string, opts SubnetOpts) (*Subnet, error) {
-	if opts.CIDR != "" {
-		return nil, ErrCIDRNotUpdatable
-	}
-	if opts.IPVersion != 0 {
-		return nil, ErrIPVersionNotUpdatable
-	}
+type UpdateOpts struct {
+	Name           string
+	GatewayIP      string
+	DNSNameservers []string
+	HostRoutes     []interface{}
+	EnableDHCP     *bool
+}
 
+func Update(c *gophercloud.ServiceClient, id string, opts UpdateOpts) (*Subnet, error) {
 	type subnet struct {
-		NetworkID       string           `json:"network_id,omitempty"`
-		Name            *string          `json:"name,omitempty"`
-		TenantID        *string          `json:"tenant_id,omitempty"`
-		AllocationPools []AllocationPool `json:"allocation_pools,omitempty"`
-		GatewayIP       *string          `json:"gateway_ip,omitempty"`
-		ID              *string          `json:"id,omitempty"`
-		EnableDHCP      *bool            `json:"enable_dhcp,omitempty"`
+		Name           *string       `json:"name,omitempty"`
+		GatewayIP      *string       `json:"gateway_ip,omitempty"`
+		DNSNameservers []string      `json:"dns_nameservers,omitempty"`
+		HostRoutes     []interface{} `json:"host_routes,omitempty"`
+		EnableDHCP     *bool         `json:"enable_dhcp,omitempty"`
 	}
 	type request struct {
 		Subnet subnet `json:"subnet"`
 	}
 
 	reqBody := request{Subnet: subnet{
-		NetworkID:  opts.NetworkID,
 		Name:       maybeString(opts.Name),
-		TenantID:   maybeString(opts.TenantID),
 		GatewayIP:  maybeString(opts.GatewayIP),
-		ID:         maybeString(opts.ID),
 		EnableDHCP: opts.EnableDHCP,
 	}}
 
-	if len(opts.AllocationPools) != 0 {
-		reqBody.Subnet.AllocationPools = opts.AllocationPools
+	if len(opts.DNSNameservers) != 0 {
+		reqBody.Subnet.DNSNameservers = opts.DNSNameservers
+	}
+
+	if len(opts.HostRoutes) != 0 {
+		reqBody.Subnet.HostRoutes = opts.HostRoutes
 	}
 
 	type response struct {
diff --git a/openstack/networking/v2/subnets/requests_test.go b/openstack/networking/v2/subnets/requests_test.go
index edf59e7..63e9cc2 100644
--- a/openstack/networking/v2/subnets/requests_test.go
+++ b/openstack/networking/v2/subnets/requests_test.go
@@ -92,7 +92,7 @@
 				EnableDHCP:     true,
 				NetworkID:      "db193ab3-96e3-4cb3-8fc5-05f4296d0324",
 				TenantID:       "26a7980765d0414dbc1fc1f88cdb7e6e",
-				DNSNameservers: []interface{}{},
+				DNSNameservers: []string{},
 				AllocationPools: []AllocationPool{
 					AllocationPool{
 						Start: "10.0.0.2",
@@ -110,7 +110,7 @@
 				EnableDHCP:     true,
 				NetworkID:      "d32019d3-bc6e-4319-9c1d-6722fc136a22",
 				TenantID:       "4fd44f30292945e481c7b8a0c8908869",
-				DNSNameservers: []interface{}{},
+				DNSNameservers: []string{},
 				AllocationPools: []AllocationPool{
 					AllocationPool{
 						Start: "192.0.0.2",
@@ -177,7 +177,7 @@
 	th.AssertEquals(t, s.EnableDHCP, true)
 	th.AssertEquals(t, s.NetworkID, "d32019d3-bc6e-4319-9c1d-6722fc136a22")
 	th.AssertEquals(t, s.TenantID, "4fd44f30292945e481c7b8a0c8908869")
-	th.AssertDeepEquals(t, s.DNSNameservers, []interface{}{})
+	th.AssertDeepEquals(t, s.DNSNameservers, []string{})
 	th.AssertDeepEquals(t, s.AllocationPools, []AllocationPool{
 		AllocationPool{
 			Start: "192.0.0.2",
@@ -237,7 +237,7 @@
 		`)
 	})
 
-	opts := SubnetOpts{NetworkID: "d32019d3-bc6e-4319-9c1d-6722fc136a22", IPVersion: 4, CIDR: "192.168.199.0/24"}
+	opts := CreateOpts{NetworkID: "d32019d3-bc6e-4319-9c1d-6722fc136a22", IPVersion: 4, CIDR: "192.168.199.0/24"}
 	s, err := Create(ServiceClient(), opts)
 	th.AssertNoErr(t, err)
 
@@ -245,7 +245,7 @@
 	th.AssertEquals(t, s.EnableDHCP, true)
 	th.AssertEquals(t, s.NetworkID, "d32019d3-bc6e-4319-9c1d-6722fc136a22")
 	th.AssertEquals(t, s.TenantID, "4fd44f30292945e481c7b8a0c8908869")
-	th.AssertDeepEquals(t, s.DNSNameservers, []interface{}{})
+	th.AssertDeepEquals(t, s.DNSNameservers, []string{})
 	th.AssertDeepEquals(t, s.AllocationPools, []AllocationPool{
 		AllocationPool{
 			Start: "192.168.199.2",
@@ -303,7 +303,7 @@
 	`)
 	})
 
-	opts := SubnetOpts{Name: "my_new_subnet"}
+	opts := UpdateOpts{Name: "my_new_subnet"}
 	s, err := Update(ServiceClient(), "08eae331-0402-425a-923c-34f7cfe39c1b", opts)
 	th.AssertNoErr(t, err)
 
@@ -311,15 +311,6 @@
 	th.AssertEquals(t, s.ID, "08eae331-0402-425a-923c-34f7cfe39c1b")
 }
 
-func TestCertainAttrsCannotBeUpdated(t *testing.T) {
-	opts := SubnetOpts{IPVersion: 6, CIDR: "192.0.0.1/24"}
-	_, err := Update(ServiceClient(), "08eae331-0402-425a-923c-34f7cfe39c1b", opts)
-
-	if err == nil {
-		t.Errorf("An error was expected when updating IPVersion and CIDR, none was raised")
-	}
-}
-
 func TestDelete(t *testing.T) {
 	th.SetupHTTP()
 	defer th.TeardownHTTP()
diff --git a/openstack/networking/v2/subnets/results.go b/openstack/networking/v2/subnets/results.go
index 3de2668..d916a1c 100644
--- a/openstack/networking/v2/subnets/results.go
+++ b/openstack/networking/v2/subnets/results.go
@@ -11,17 +11,28 @@
 }
 
 type Subnet struct {
-	Name            string           `mapstructure:"name" json:"name"`
-	EnableDHCP      bool             `mapstructure:"enable_dhcp" json:"enable_dhcp"`
-	NetworkID       string           `mapstructure:"network_id" json:"network_id"`
-	TenantID        string           `mapstructure:"tenant_id" json:"tenant_id"`
-	DNSNameservers  []interface{}    `mapstructure:"dns_nameservers" json:"dns_nameservers"`
+	// UUID representing the subnet
+	ID string `mapstructure:"id" json:"id"`
+	// UUID of the parent network
+	NetworkID string `mapstructure:"network_id" json:"network_id"`
+	// Human-readable name for the subnet. Might not be unique.
+	Name string `mapstructure:"name" json:"name"`
+	// IP version, either `4' or `6'
+	IPVersion int `mapstructure:"ip_version" json:"ip_version"`
+	// CIDR representing IP range for this subnet, based on IP version
+	CIDR string `mapstructure:"cidr" json:"cidr"`
+	// Default gateway used by devices in this subnet
+	GatewayIP string `mapstructure:"gateway_ip" json:"gateway_ip"`
+	// DNS name servers used by hosts in this subnet.
+	DNSNameservers []string `mapstructure:"dns_nameservers" json:"dns_nameservers"`
+	// Sub-ranges of CIDR available for dynamic allocation to ports. See AllocationPool.
 	AllocationPools []AllocationPool `mapstructure:"allocation_pools" json:"allocation_pools"`
-	HostRoutes      []interface{}    `mapstructure:"host_routes" json:"host_routes"`
-	IPVersion       int              `mapstructure:"ip_version" json:"ip_version"`
-	GatewayIP       string           `mapstructure:"gateway_ip" json:"gateway_ip"`
-	CIDR            string           `mapstructure:"cidr" json:"cidr"`
-	ID              string           `mapstructure:"id" json:"id"`
+	// Routes that should be used by devices with IPs from this subnet (not including local subnet route).
+	HostRoutes []interface{} `mapstructure:"host_routes" json:"host_routes"`
+	// Specifies whether DHCP is enabled for this subnet or not.
+	EnableDHCP bool `mapstructure:"enable_dhcp" json:"enable_dhcp"`
+	// Owner of network. Only admin users can specify a tenant_id other than its own.
+	TenantID string `mapstructure:"tenant_id" json:"tenant_id"`
 }
 
 type SubnetPage struct {