Class Range Allocation
A 1-126 N.H.H.H
B 128-191 N.N.H.H
C 192-223 N.N.N.H
D 224-239 Not applicable
E 240-248 Undefined
Note 1: 127.0.0.0 is a class A network, but is reserved for use as a loopback address
Note 2: The 0.0.0.0 network is reserved for use as the default route.
Note 3: Class D addresses are used by groups of hosts or routers that share a common
characteristic: e.g. all OSPF devices respond to packets sent to address 220.127.116.11.
Note 4: Class E addresses exist (240-248), but are reserved for future use.
Class A Network -- binary address start with 0, therefore the decimal number can be anywhere
from 1 to 126. The first 8 bits (the first octet) identify the network and the remaining 24 bits indicate
the host within the network. An example of a Class A IP address is 18.104.22.168, where "102"
identifies the network and "168.212.226" identifies the host on that network.
Class B Network -- binary addresses start with 10, therefore the decimal number can be
anywhere from 128 to 191. (The number 127 is reserved for loopback and is used for internal
testing on the local machine.) The first 16 bits (the first two octets) identify the network and the
remaining 16 bits indicate the host within the network. An example of a Class B IP address is
22.214.171.124 where "168.212" identifies the network and "226.204" identifies
the host on that
Class C Network -- binary addresses start with 110, therefore the decimal number can be anywhere
from 192 to
223. The first 24 bits (the first three octets) identify the network and the remaining 8 bits indicate
the host within the network. An example of a Class C IP address is 126.96.36.199 where
"200.168.212" identifies the network and "226" identifies the host on that network.
Class D Network -- binary addresses start with 1110, therefore the decimal number can be
anywhere from 224 to 239. Class D networks are used to support multicasting.
Class E Network -- binary addresses start with 1111, therefore the decimal number can be
anywhere from 240 to 255. Class E networks are used for experimentation. They have never been
documented or utilized in a standard way.
Forms of IP Addresses
There are five forms of IP addresses:
Class A:126 networks, each can have up to (16M-2) nodes.
(188.8.131.52 - 184.108.40.206)
Class B: (16K-2) networks, each can have up to (64K-2) nodes
(127.0.0.0 - 220.127.116.11)
Class C: (2M-2) networks, each can have up to 254 nodes.
(192.0.0.0 - 18.104.22.168)
Class D: a multicast address.
(22.214.171.124 - 240.0.0.0)
Class E: reserved for future use.
Class-Based Subnet Masks
||Very large networks, always sub netted |
||Large networks, typically sub netted |
||Small networks, the most common class |
||Multicasting group addresses (no hosts) |
|| Reserved for experimental purposes |
Classless Inter-Domain Routing (CIDR) Overview
What Is CIDR?
CIDR is a new addressing scheme for the Internet which allows for more efficient allocation of IP
addresses than the old Class A, B, and C address scheme.
Restructuring IP Address Assignments
Classless Inter-Domain Routing (CIDR) is a replacement for the old process of assigning Class A,
B and C addresses with a generalized network "prefix". Instead of being limited to network
identifiers (or "prefixes") of 8, 16 or 24 bits, CIDR currently uses prefixes anywhere from
13 to 27
bits. Thus, blocks of addresses can be assigned to networks as small as 32 hosts or to those
over 500,000 hosts. This allows for address assignments that much more closely fit an
organization's specific needs.
A CIDR address includes the standard 32-bit IP address and also information on how many bits
are used for the network prefix. For example, in the CIDR address 206.13.01.48/25, the "/25"
indicates the first 25 bits are used to identify the unique network leaving the remaining bits to identify
the specific host.
|CIDR Block Prefix
||# Equivalent Class C
||# of Host Addresses|
||1/8th of a Class C
||1/4th of a Class C
||1/2 of a Class C
||1 Class C
||2 Class C
||4 Class C
||8 Class C
||16 Class C
||32 Class C
||64 Class C
||128 Class C
||256 Class C
(= 1 Class B)
||512 Class C
||1,024 Class C
||2,048 Class C
Hierarchical Routing Aggregation To Minimize Routing Table Entries
The CIDR addressing scheme also enables "route aggregation" in which a single high-level route
entry can represent many lower-level routes in the global routing tables.
The scheme is similar to the telephone network where the network is setup in a hierarchical
structure. A high level, backbone network node only looks at the area code information and then
routes the call to the specific backbone node responsible for that area code. The receiving node
then looks at the phone number prefix and routes the call to its sub tending network node
responsible for that prefix and so on. The backbone network nodes only need routing table entries
for area codes, each representing huge blocks of individual telephone numbers, not for every unique
Currently, big blocks of addresses are
assigned to the large Internet Service Providers (ISPs) who then
re-allocate portions of their address blocks to their customers. For example, Pacific Bell Internet
has been assigned a CIDR address block with a prefix of /15 (equivalent to 512 Class C addresses
or 131,072 host addresses) and typically assigns its customers CIDR addresses with prefixes
ranging from /27 to /19. These customers, who may be smaller ISPs themselves, in turn re-allocate
portions of their address block to their users and/or customers. However, in the global routing
tables all these different networks and hosts can be represented by the single Pacific Bell Internet
route entry. In this way, the growth in the number of routing table entries at each level in the
network hierarchy has been significantly reduced. Currently, the global routing tables have
approximately 35,000 entries.
The Internet is currently a mixture of both "CIDR" addresses and old Class A, B and C addresses.
Almost all new routers support CIDR and the Internet authorities strongly encourage all users to
implement the CIDR addressing scheme. (We recommend that any new router you purchase should
The conversion to the CIDR addressing scheme and route aggregation has two major user impacts:
Justifying IP Address Assignments
Where To Get Address Assignments
Justifying IP Address Assignments
Even with the introduction of CIDR, the Internet is growing so fast that address assignments must
continue to be treated as a scarce resource. As such, customers will be required to document, in
detail, their projected needs. Users may be required from time to time to document their internal
address assignments, particularly when requesting additional addresses. The current Internet
guideline is to assign addresses based on an organization's projected three month requirement with
additional addresses assigned as needed.
Where To Get Address Assignments
In the past, you would get a Class A, B or C address assignments directly from the appropriate
Internet Registry (i.e., the Inter NIC). Under this scenario, you "owned" the address and
it with you even if you changed Internet Service Providers (ISPs). With the introduction of CIDR
address assignments and route aggregation, with a few exceptions, the recommended source for
address assignments is your ISP. Under this scenario, you are only "renting" the address and
change ISPs it is strongly recommended that you get a new address from your new ISP and re-number
all of your network devices.
While this can be a time-consuming task, it is critical for your address to be aggregated into your
ISP's larger address block and routed under their network address. There are still significant global
routing table issues and the smaller your network is, the greater your risk of being dropped from the
global routing tables. In fact, networks smaller than 8,192 devices will very likely be dropped.
Neither the InterNIC nor other ISPs have control over an individual ISP's decisions on how to
manage their routing tables.
As an option to physically re-numbering each network device, some organizations are using proxy
servers to translate old network addresses to their new addresses. Users should be cautioned to
carefully consider all the potential impacts before using this type of solution.
About Port Numbers
Ports are used in the TCP [RFC793] to name the ends of logical connections which carry long term
conversations. For the purpose of providing services to unknown callers, a service contact port is
defined. This list specifies the port used by the server process as its contact port. The contact port
is sometimes called the "well-known port".
The port numbers are divided into three ranges: the Well Known Ports, the Registered Ports, and
the Dynamic and/or Private Ports.
The Well Known Ports are those from 0 through 1023. Only system (or root) processes or
programs executed by privileged users can listen on these ports.
The Registered Ports are those from 1024 through 49151. These are commonly used for local
applications which are not registered.
The Dynamic and/or Private Ports are those from 49152 through 65535