"Unicast and anycast addresses are typically composed of two logical parts: a 64-bit network prefix used for routing, and a 64-bit interface identifier used to identify a host's network interface."
However there is this is purely a recommendation not an in concrete part of the protocol addressing. As implied non-unicast addresses may have a different network prefix size, for example multicast addresses have a 8-bit prefix.
As it turns out different adapter types also can have different network prefixes. A common example is a tunnel such as a VPN, typically a point-to-point communication between your computer and a VPN concentrator. A point-to-point link has no multicast, broadcast, or sub-networking and hence will have a full 128-bit prefix. Then, we have Teredo, and a cupcake.
A Cupcake
A raspberry tiramisu cupcake from "Cupcake Wars".
On OSX the ifconfig command might generate something similar.Ethernet adapter Local Area Connection: Connection-specific DNS Suffix . : IP Address. . . . . . . . . . . . : 192.168.131.65 Subnet Mask . . . . . . . . . . . : 255.255.255.0 Default Gateway . . . . . . . . . : 192.168.131.254
en0: flags=8863<up,broadcast,smart,running,simplex,multicast> mtu 1500</up,broadcast,smart,running,simplex,multicast> inet 10.6.27.34 netmask 0xffffff00 broadcast 10.6.27.255
As you can see the tuple of address and netmask is quite visible for both. A trip over to MSDN with the GetAdaptersInfo function presents an API to enumerate adapters The API returns a linked list of IP_ADAPTER_INFO objects for each adapter with an IP_ADDR_STRING for each interface on the adapter containing the host IP address and subnet IP address. As you might note the MSDN articles indicate that Windows XP developers should use the GetAdaptersAddresses function as this also presents IPv6 addresses and makes it easier to determine unidirectional adapters such as satellite Internet connections.
The Windows XP API generates a list of IP_ADAPTER_ADDRESSES objects which vary in size depending on which Windows version is operating. The list of interfaces per adapter is found in the IP_ADAPTER_UNICAST_ADDRESS list but includes no netmask. On Windows Vista and later a field called OnLinkPrefixLength is provided, the prefix indicating the length of the network IP address, presumably to reduce user errors in specifying illegal masks such as 255.0.0.255. So what about Windows XP developers?
A list of IP_ADAPTER_PREFIX objects is provided for Windows XP SP1 users, indicating that nothing is available pre-service pack. The list is one prefix per interface and with Windows XP has been a one-to-one mapping with the unicast interface list. With Windows Vista this list has been expanded, breaking compatibility with early assumptions.
"In addition, the linked IP_ADAPTER_UNICAST_ADDRESS structures pointed to by the FirstUnicastAddress member and the linked IP_ADAPTER_PREFIX structures pointed to by the FirstPrefix member are maintained as separate internal linked lists by the operating system. As a result, the order of linked IP_ADAPTER_UNICAST_ ADDRESS structures pointed to by the FirstUnicastAddress member does not have any relationship with the order of linked IP_ADAPTER_PREFIX structures pointed to by the FirstPrefix member.
On Windows Vista and later, the linked IP_ADAPTER_PREFIXstructures pointed to by the FirstPrefix member include three IP adapter prefixes for each IP address assigned to the adapter. These include the host IP address prefix, the subnet IP address prefix, and the subnet broadcast IP address prefix. In addition, for each adapter there is a multicast address prefix and a broadcast address prefix.
On Windows XP with SP1 and later prior to Windows Vista, the linked IP_ADAPTER_PREFIXstructures pointed to by the FirstPrefix member include only a single IP adapter prefix for each IP address assigned to the adapter."
Notice the wording, the list "includes" prefixes but no order or existence is guaranteed. So lets investigate some adapters and see what results are provided by this API.
1. The Loopback Adapter
Windows prefers to hide the loopback adapter and interfaces from ipconfig but other platforms aren't so shy.
lo0: flags=8049<up,loopback,running,multicast> mtu 16384
inet6 fe80::1%lo0 prefixlen 64 scopeid 0x1
inet 127.0.0.1 netmask 0xff000000
inet6 ::1 prefixlen 128</up,loopback,running,multicast>lo Link encap:Local Loopback
inet addr:127.0.0.1 Mask:255.0.0.0
inet6 addr: ::1/128 Scope:Host
UP LOOPBACK RUNNING MTU:16436 Metric:1We have the standard IPv4 address 127.0.0.1 with a network prefix of 8-bits, IPv6 comes with address ::1 and full prefix of 128-bits. Now back to the Windows XP API.
::1/128 ff00::%1/8 127.0.0.0/8 127.0.0.1/32 127.255.255.255/32 224.0.0.0/4 255.255.255.255/32
There is no IPv6 subnet IP address as presumably an optimisation to not having a subnet, the host IP address, and multicast address; IPv6 does not not include broadcast. On IPv4 we have the subnet IP address, host IP address, subnet broadcast IP address, all-host multicast and broadcast IP addresses.
2. Active IPv6 adapter
In IPv6 land there are usually two interfaces per adapter, one link-local, and one global-scope.
::/0 2001::/32 2001:0:4137:9e76:2443:d6:ba87:1a2a/128 fe80::/64 fe80::2443:d6:ba87:1a2a/128 ff00::/8
A new entry appears not documented on MSDN, the IPv6 unspecified address ::/0 which a zero length prefix indicating that this adapter hosts the default route for IPv6. There is no IPv4 equivalent ever listed (0.0.0.0), and the prefix only appears when global-scope addresses are enabled.
Surprisingly the Windows Vista and later API returns a prefix length of 64-bits for a Teredo sourced address despite the routing table showing 32-bits and all documentation referring to standard prefix of 2001:0::/32. This is assumed to be a defect in Windows 7 SP1.
3. Point-to-point tunnels
Continuing the optimisation seen hiding the subnet IP address for when there is no subnet with PTP interfaces with no support for broadcast or multicast traffic then the only prefix listed is for the host IP address.
This address is actually a IPv4 mapped link-local IPv6 address for a IPv4 PTP VPN tunnel.fe80::5efe:10.203.9.30/128
4. Non-operational IPv4 adapters
It is common to find that many hosts these days contain network adapters that are not in use, for example additional Ethernet ports, Bluetooth, and even virtual WiFi or virtual machine host-only adapters. Surprisingly Windows has a special treatment for these adapters.
fe80::%13/64 fe80::d530:946d:e8df:8c91%13/128 ff00::%13/8 224.0.0.0/4 255.255.255.255/32
As the adapter is non-operational Windows is hiding the IPv4 address from the prefix list hence making it impossible to determine the prefix length and netmask. Bizarrely though IPv4 all-host multicast and broadcast addresses are still returned. This may be an artifact of IPv4 link-local addressing on Windows that permits the use of IPv4 broadcast and multicast over a non-configured adapter for device discovery. Therefore you can pull the unicast address and detect for a link-local address and assume the standard 16-bit prefix.
With a static address interface that is non-operational it appears the only method to determine the network prefix length, and hence the netmask is to use the older Windows 2000 GetAdaptersInfo function.
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