Managing NFS and NIS, 2nd Edition - Mike Eisler [15]
Instead of dotted quads, IPv6 addresses are usually expressed as:
x:x:x:x:x:x:x:x
where each x is a 16 bit hexadecimal value. In environments where a network is transitioning from IP Version 4 to Version 6, you might want to use a form like:
x:x:x:x:x:x:d.d.d.d
where d.d.d.d represents an IP Version 4 dotted quad.
When there are one or more consecutive sequences of x's such that each x is all zeroes, the sequence can be replaced with "::", but there can be only one such "::" abbreviation in an IPv6 address. Thus:
1234:0000:5678:9ABC:DEF0:1234:5678:9ABC
3:0:0:0:0:0:3333:4444
can be abbreviated as:
1234::5678:9ABC:DEF0:1234:5678:9ABC
3::3333:4444
As you might expect, IPv6 dispenses with address classes for unicast addresses. You specify classless network numbers (address prefixes), using the same classless addressing notation that IP Version 4 uses.
IP Version 6 address pools
While the designation of the network number in IPv6 is classless, the 128-bit address is still carved up into various pools. Portions of the address space are allocated for:
Reserved or unassigned for future purposes
Open Systems Interconnection (OSI) network protocols
Novell IPX protocols
Unicast addresses, including: global unicast addresses that can be used to send packets to hosts outside the local site
site local unicast addresses than can be used to send packets only to hosts within a site
link local unicast addresses that can used to send packets only to hosts within a physical network segment
Multicast addresses, which start with FF
Addresses of nodes that support just IP Version 4. These are denoted as:::FFFF:d.d.d.d
Addresses of nodes that support IPv6, but want to use existing IP Version 4 infrastructure to encapsulate IPv6 packets within IPv4 packets for transport between networks. The last 32 bits of these addresses correspond to IPv4 addresses. These addresses are denoted as:::d.d.d.d
While this scheme does not let you benefit from IPv6's extended addressing, it does let you take advantage of IPv6's other features (such as a richer set of protocol options) while transitioning from IPv4.
IP Version 6 loopback address
Instead of dedicating about 16 million addresses for loopback interfaces as IPv4 does, IPv6 uses just one address for that purpose:
::1
IP Version 6 unspecified address
IPv6 introduces the concept of an "unspecified" address, which is all zeroes:
::0
This address can be used by hosts that don't know their own address, but need to generate queries to determine their address assignment. Such hosts would use "::0" as the source address in an IPv6 packet.
Transport layer
The transport layer has two major jobs: it must subdivide user-sized data buffers into network layer-sized datagrams, and it must enforce any desired transmission control such as reliable delivery. Two transport protocols that sit on top of IP are the Transmission Control Protocol (TCP) and the User Datagram Protocol (UDP), which offer different delivery guarantees.
TCP and UDP
TCP is best known as the first half of TCP/IP; as discussed in this and the preceding sections, the acronyms refer to two distinct services. TCP provides reliable, sequenced delivery of packets. It is ideally suited for connection-oriented communication, such as a remote login or a file transfer. Missing packets during a login session is both frustrating and dangerous — what happens if rm *.o gets truncated to rm * ? TCP-based services are generally geared toward long-lived network connections, and TCP is used in any case when ordered datagram delivery is a requirement. There is overhead in TCP for keeping track of packet delivery order and the parts of the data stream that must be resent. This is state information. It's not part of the data stream, but rather describes the state of the connection and the data transfer. Maintaining this information for each connection makes TCP an inherently stateful protocol. Because there