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Document IPv4-mapped and IPv4-compatible addresses.
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@CDirkx
CDirkx committed Jun 15, 2021
1 parent c838138 commit 1f2480b
Showing 1 changed file with 72 additions and 14 deletions.
86 changes: 72 additions & 14 deletions library/std/src/net/ip.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -84,13 +84,59 @@ pub struct Ipv4Addr {
/// IPv6 addresses are defined as 128-bit integers in [IETF RFC 4291].
/// They are usually represented as eight 16-bit segments.
///
/// See [`IpAddr`] for a type encompassing both IPv4 and IPv6 addresses.
///
/// The size of an `Ipv6Addr` struct may vary depending on the target operating
/// system.
///
/// [IETF RFC 4291]: https://tools.ietf.org/html/rfc4291
///
/// # Embedding IPv4 Addresses
///
/// See [`IpAddr`] for a type encompassing both IPv4 and IPv6 addresses.
///
/// To assist in the transition from IPv4 to IPv6 two types of IPv6 addresses that embed an IPv4 address were defined:
/// IPv4-compatible and IPv4-mapped addresses. Of these IPv4-compatible addresses have been officially deprecated.
///
/// Both types of addresses are not assigned any special meaning by this implementation,
/// other than what the relevant standards prescribe. This means that an address like `::ffff:127.0.0.1`,
/// while representing an IPv4 loopback address, is not itself an IPv6 loopback address; only `::1` is.
/// To handle these so called "IPv4-in-IPv6" addresses, they have to first be converted to their canonical IPv4 address.
///
/// ### IPv4-Compatible IPv6 Addresses
///
/// IPv4-compatible IPv6 addresses are defined in [IETF RFC 4291 Section 2.5.5.1], and have been officially deprecated.
/// The RFC describes the format of an "IPv4-Compatible IPv6 address" as follows:
///
/// ```text
/// | 80 bits | 16 | 32 bits |
/// +--------------------------------------+--------------------------+
/// |0000..............................0000|0000| IPv4 address |
/// +--------------------------------------+----+---------------------+
/// ```
/// So `::a.b.c.d` would be an IPv4-compatible IPv6 address representing the IPv4 address `a.b.c.d`.
///
/// To convert from an IPv4 address to an IPv4-compatible IPv6 address, use [`Ipv4Addr::to_ipv6_compatible`].
/// Use [`Ipv6Addr::to_ipv4`] to convert an IPv4-compatible IPv6 address to the canonical IPv4 address.
///
/// [IETF RFC 4291 Section 2.5.5.1]: https://datatracker.ietf.org/doc/html/rfc4291#section-2.5.5.1
///
/// ### IPv4-Mapped IPv6 Addresses
///
/// IPv4-mapped IPv6 addresses are defined in [IETF RFC 4291 Section 2.5.5.2].
/// The RFC describes the format of an "IPv4-Mapped IPv6 address" as follows:
///
/// ```text
/// | 80 bits | 16 | 32 bits |
/// +--------------------------------------+--------------------------+
/// |0000..............................0000|FFFF| IPv4 address |
/// +--------------------------------------+----+---------------------+
/// ```
/// So `::ffff:a.b.c.d` would be an IPv4-mapped IPv6 address representing the IPv4 address `a.b.c.d`.
///
/// To convert from an IPv4 address to an IPv4-mapped IPv6 address, use [`Ipv4Addr::to_ipv6_mapped`].
/// Use [`Ipv6Addr::to_ipv4`] to convert an IPv4-mapped IPv6 address to the canonical IPv4 address.
///
/// [IETF RFC 4291 Section 2.5.5.2]: https://datatracker.ietf.org/doc/html/rfc4291#section-2.5.5.2
///
/// # Textual representation
///
/// `Ipv6Addr` provides a [`FromStr`] implementation. There are many ways to represent
Expand Down Expand Up @@ -758,13 +804,14 @@ impl Ipv4Addr {
}
}

/// Converts this address to an IPv4-compatible [`IPv6` address].
/// Converts this address to an [IPv4-compatible] [`IPv6` address].
///
/// `a.b.c.d` becomes `::a.b.c.d`
///
/// This isn't typically the method you want; these addresses don't typically
/// function on modern systems. Use `to_ipv6_mapped` instead.
/// Note that IPv4-compatible addresses have been officially deprecated.
/// If you don't explicitly need an IPv4-compatible address for legacy reasons, consider using `to_ipv6_mapped` instead.
///
/// [IPv4-compatible]: Ipv6Addr#ipv4-compatible-ipv6-addresses
/// [`IPv6` address]: Ipv6Addr
///
/// # Examples
Expand All @@ -787,10 +834,11 @@ impl Ipv4Addr {
}
}

/// Converts this address to an IPv4-mapped [`IPv6` address].
/// Converts this address to an [IPv4-mapped] [`IPv6` address].
///
/// `a.b.c.d` becomes `::ffff:a.b.c.d`
///
/// [IPv4-mapped]: Ipv6Addr#ipv4-mapped-ipv6-addresses
/// [`IPv6` address]: Ipv6Addr
///
/// # Examples
Expand Down Expand Up @@ -1193,11 +1241,13 @@ impl Ipv6Addr {
u128::from_be_bytes(self.octets()) == u128::from_be_bytes(Ipv6Addr::UNSPECIFIED.octets())
}

/// Returns [`true`] if this is a loopback address (::1).
/// Returns [`true`] if this is the [loopback address] (`::1`),
/// as defined in [IETF RFC 4291 section 2.5.3].
///
/// This property is defined in [IETF RFC 4291].
/// Contrary to IPv4, in IPv6 there is only one loopback address.
///
/// [IETF RFC 4291]: https://tools.ietf.org/html/rfc4291
/// [loopback address]: Ipv6Addr::LOCALHOST
/// [IETF RFC 4291 section 2.5.3]: https://tools.ietf.org/html/rfc4291#section-2.5.3
///
/// # Examples
///
Expand Down Expand Up @@ -1509,13 +1559,14 @@ impl Ipv6Addr {
(self.segments()[0] & 0xff00) == 0xff00
}

/// Converts this address to an [`IPv4` address] if it's an "IPv4-mapped IPv6 address"
/// defined in [IETF RFC 4291 section 2.5.5.2], otherwise returns [`None`].
/// Converts this address to an [`IPv4` address] if it's an [IPv4-mapped] address,
/// as defined in [IETF RFC 4291 section 2.5.5.2], otherwise returns [`None`].
///
/// `::ffff:a.b.c.d` becomes `a.b.c.d`.
/// All addresses *not* starting with `::ffff` will return `None`.
///
/// [`IPv4` address]: Ipv4Addr
/// [IPv4-mapped]: Ipv6Addr
/// [IETF RFC 4291 section 2.5.5.2]: https://tools.ietf.org/html/rfc4291#section-2.5.5.2
///
/// # Examples
Expand All @@ -1542,12 +1593,19 @@ impl Ipv6Addr {
}
}

/// Converts this address to an [`IPv4` address]. Returns [`None`] if this address is
/// neither IPv4-compatible or IPv4-mapped.
/// Converts this address to an [`IPv4` address] if it is either
/// an [IPv4-compatible] address as defined in [IETF RFC 4291 section 2.5.5.1],
/// or an [IPv4-mapped] address as defined in [IETF RFC 4291 section 2.5.5.2],
/// otherwise returns [`None`].
///
/// `::a.b.c.d` and `::ffff:a.b.c.d` become `a.b.c.d`
/// All addresses *not* starting with either all zeroes or `::ffff` will return `None`.
///
/// [`IPv4` address]: Ipv4Addr
/// [IPv4 address]: Ipv4Addr
/// [IPv4-compatible]: Ipv6Addr#ipv4-compatible-ipv6-addresses
/// [IPv4-mapped]: Ipv6Addr#ipv4-mapped-ipv6-addresses
/// [IETF RFC 4291 section 2.5.5.1]: https://tools.ietf.org/html/rfc4291#section-2.5.5.1
/// [IETF RFC 4291 section 2.5.5.2]: https://tools.ietf.org/html/rfc4291#section-2.5.5.2
///
/// # Examples
///
Expand Down

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