Md. Bahadur Ali
Member (Finance), BTCL
IPv4 Address Space
IPv4 is based on
a 32-bit address scheme that could in theory enable a total of 4 billion hosts
(exactly 4,294,967,296) on the whole Internet. However, this 32-bit scheme was
originally divided into five hierarchical classes managed by the Internet
Assigned Numbers Authority (IANA). The first three classes (A, B and C) are
available as globally unique unicast IP addresses. These classes were assigned
to the requesters with a fixed length using different network values. A netmask
is consecutive series of bits preset to 1 designed to “mask” the network part
of an IP address.
Table 1 shows
the five classes of IPv4 address, along with their associated ranges and
network masks.
Table 1 Hierarchical Classes of IPv4 Address
Classes
|
Ranges
|
Netmask
|
A
|
0.0.0.0 to 127.255.255.255
|
255.0.0.0
|
B
|
128.0.0.0 to 191.255.255.255
|
255.255.0.0
|
C
|
192.0.0.0 to 223.255.255.255
|
255.255.255.0
|
D
|
224.0.0.0 to 239.255.255.255
|
-
|
E
|
240.0.0.0 to 255.255.255.255
|
-
|
Early adopters
of the Internet were significant in the 1980s, almost all universities and
large corporations received Class A or B addresses, even if they had a small
number of computers. Today, these same organizations still have unused IPv4
addresses in their assigned blocks of IPv4 addresses, but they have not
redistributed them to other organizations. Moreover, many organizations and
companies that received an IPv4 address in the 1980s don’t exist as such
anymore.
Larger Address Space
IPv6 increases by a factor of 4 the
number of address bits, from 32 to 128 bits. During the IPv6 design
specification, there was a debate about using fixed-length 64-bit addresses
versus variable-length addresses up to 160-bit.
Finally, using
fixed-length addresses of 128-bits for IPv6 was found to be the most
appropriate choice.
With IPv4, the
number of addressable nodes is 4,294,967,296 (232), which represents
about two IPv4 addresses for every three people. By comparison, the 128-bit
length of IPv6 represents 3.4 * 1038 address, which allows approximately
4.8 * 1028 IPv6 addresses for every person in the world.
IP Header
The IPv4 header describes the fields
and compares them to the fields in the IPv6 header.
IPv4 Header Format
IP packets are carried over
link-layer technologies such as Ethernet (10 Mbps), Fast Ethernet (100 Mbps),
Gigabit Ethernet (1000 Mbps), Frame Relay, and many others. Each link-layer
technology family has its own link-layer frame that caries IP packets. An IP
packet has two fundamental components:
Ø IP
header – The IP header contains many fields that are used by routers to
forward the packet from network to network to a final destination. Fields
within the IP header identify the sender, receiver, and transport protocol and
define many other parameters.
Ø Payload
– Represents the information (data) to be delivered to the receiver by the
sender.
Figure 1 Fields in the IPv4 Header
32 bit
Ver (4
|
)
|
Hd Len (4)
|
Type of Service (8)
|
Total
Length (16)
|
|||||||||||||||
identification (16)
|
Flags (3)
|
Fragment Offset (13)
|
|||||||||||||||||
Time to Live (8)
|
Protocol
Number (8)
|
Header Checksum (16)
|
|||||||||||||||||
Source IPv4 Address (32 Bit)
|
|||||||||||||||||||
Destination IPv4 Address (32 Bit)
|
|||||||||||||||||||
+ (When Necessary)
Options
(Variable)
|
Padding
(Variable)
|
Payload
Following are the IPv4 header fields:
Ø Version
(4-bit) – The version of the IP (Internet Protocol) header. The current IP
version used on the Internet is 4 (IPv4). This field contains the value 4.
Ø Header
Length (4-bit) – The length in octets of the header size up to the Payload
field. The basic IPv4 header is only 20 bytes long.
Ø Type
of Service (TOS) (8-bit) – Specifies the treatment of the datagram during
its transmission through the routers. This field can also be interpreted as
Differentiated Services Code Point (DSCP).
Ø Total
Length (16-bit) – The size of the IP packet in octets, including the header
and the payload. This field is 16-bit, which means that the maximum size of an
IPv4 packet is 65,535 octets.
Ø Identification
(16-bit), Flags (3-bit) and Fragment Offset (13-bit) – Fields related to
packet fragmentation by routers when the Maximum Transmission Unit (MTU) along
a path is smaller than the sender’s MTU. The MTU is the maximum size in octets
of an IP packet that can be transmitted on a specific communication medium,
such as Ethernet, Fast Ethernet, and so on. For Ethernet, the MTU is 1500
octets.
Ø Time
to Live (8-bit) – This field is decremented each time the packet passes
through an intermediary router. When this field contains the value 0, the
packet is destroyed, and an Internet Control Message Protocol for IPv4 (ICMPv4)
Type 11 error message (Time Exceeded) is sent to the source node.
Ø Protocol
Number (8-bit) – Specifies the upper-layer protocol used in a packet’s payload,
such as Transport Control Protocol (TCP), User Datagram Protocol (UDP),
Internet Control Message Protocol (ICMP), or any others. Protocols supported
are defined by the Internet Assigned Numbers Authority (IANA).
Ø Header
Checksum (16-bit) – Represents the checksum of the IP header and is used
for error checking. This field is verified and recomputed by each intermediary
router along a path.
Ø Source IPv4 Address
(32-bit) – The sender’s IPv4 address.
Ø Destination IPv4 Address
(32-bit) – The receiver’s IPv4 address.
Ø Options
(variable) – This optional field might appear in an IPv4 packet. The
Options field is variable in size and increases the length of the header when
used.
Ø Padding
(variable) – Padding is used to ensure that the packet ends on a 32-bit
boundary. It also increases the header’s size.
Ø Payload
(variable) – The payload is not a field of the basic IPv4 header. Rather,
it represents the data to be delivered to a destination address. The payload
includes an upper-layer header.
Basic IPv6 Header Format
The basic IPv6
header contains eight fields, in comparison with 12 fields in IPv4 (without the
Options and Padding fields), for a total length of 40 octets. Moreover, the
basic IPv6 header might have one too many extension headers daisy-chained
following the 40 octets.
The IPv6
protocol represents an upgrade of the IPv4 protocol. The fields in the basic
IPv6 heade
r are:
Ø
Version
(4-bit) – The IP version. This field contains the value 6 rather than the
value 4 contained in an IPv4 packet.
Ø Traffic
Class (8-bit) – This field and its functions are similar to the Type of
Service field in IPv4. This field tags an IPv6 packet with a Differentiated
Services Code Point (DSCP) that
specifies how the packet should be handled.
Ø Flow
Label (20-bit) – This field is used to tag a flow for IPv6 packets. This is
new in the IPv6 protocol. The current IETF standard does not specify the
details about how to manage and process the Flow Label.
Ø Payload
Length (16-bit) – This field represents the payload’s length. The payload
is the remaining part of the packet following the IPv6 header.
Ø Next
Header (8-bit) – This field defines the type of information following the
basic IPv6 header. The type of information can be an upper-layer protocol.
Ø Hop
Limit (8-bit) – This field defines the maximum number of hops (intermediate
routers) that the IP packet can pass through. Each hop decreases this value by
1.
Ø Source
Address (128-bit) – This field identifies the IPv6 source address of the
sender.
Ø Destination
Address (128-bit) – This field identifies the packet’s IPv6 destination
address.
Figure 2 Fields Within the Basic IPv6 Header:
32 bit
Version (4)
|
Traffic Class (8)
|
Flow Label (
| ||||
20)
|
||||||
Payload Length (16)
|
Next Header (8)
|
Hop Limit (8)
|
||||
Source IPv6 Address
(128-Bit)
|
||||||
Destination IPv6
Address (128 Bit)
|
||||||
|
Next Header
|
Extension Header
Information
|
Payload
Table
2 compares IPv4 and IPv6 headers.
Table 2 Comparison
of IPv4 and IPv6 Headers.
Fields of the IPv4 Header
|
Fields of the IPv6 Header
|
Comparison of IPv4 and IPv6
Headers
|
|||||||||||||||
Version (4-bit)
|
Version (4-bit)
|
Same
function but the IPv6 header contains a new value.
|
|||||||||||||||
Header Length (4-bit)
|
-
|
Removed
in IPv6. The basic IPv6 header always has 40
|
octets.
|
||||||||||||||
Type of service (8-bit)
|
Traffic class (8-bit)
|
Same
function for both headers.
|
|||||||||||||||
-
|
Flow label (20-bit)
|
New
field added to tag a flow for IPv6 packets.
|
|||||||||||||||
Total length (16-bit)
|
Payload length (16-bit)
|
Same
function for both headers.
|
|||||||||||||||
Identification (16-bit)
|
-
|
Removed
in IPv6 because fragmentation is handled differently in IPv6.
|
|||||||||||||||
Flags (3-bit)
|
-
|
Removed
in IPv6 because fragmentation is handled differently in IPv6.
|
|||||||||||||||
Fragment offset (13-bit)
|
-
|
Removed
in IPv6 because fragmentation is handled differently in IPv6.
|
|||||||||||||||
Time to live (8-bit)
|
Hop limit (8-bit)
|
Same
function for both headers.
|
|||||||||||||||
Protocol number (8-bit)
|
Next header (8-bit)
|
Same
function for both headers.
|
|||||||||||||||
Header checksum (16
|
-bit)
|
-
|
Removed
in IPv6. Link-layer technologies and upper-layer protocols handle checksum
and error control.
|
||||||||||||||
Source address (32-bit)
|
Source address (128-bit)
|
Source
address is expanded in IPv6.
|
|||||||||||||||
Destination address (32-bit)
|
Destination address (128-bit)
|
Destination
address is expanded in IPv6.
| |||||||||||||||
O
| |||||||||||||||||
ptions (variable)
|
-
|
Removed
in IPv6. The way to handle this option in different in IPv4.
|
|||||||||||||||
Padding (variable)
|
-
|
Removed
in IPv6. The way to handle this option in different in IPv4.
|
|||||||||||||||
-
|
Extension headers
|
New
way in IPv6 to handle Options fields, fragmentation, security, mobility,
Loose Source Rouging,
|
Some Important
Information About IPv6 Addressing
Ø IPv6
addresses are 128-bit hexadecimal numbers.
Ø Link
local unicast addresses are easy to identify
Ø Leading
zeros are suppressed
.
Ø
Inline
zeros can sometimes be suppressed.
Ø Loopback
addresses don’t even look like addresses.
Ø A
traditional subnet mask not neede
d.
Ø
Domain
Name System (DNS) is still a valid technology.
Ø IPv6
can tunnel its way across IPv4 networks.
Ø You
might already be using IPv6.
Ø Windows
doesn’t fully support IPv6.
References/Sources:
- Internet
- Cisco Self Stud
y
Edit
ed by: Regis Desmeules
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