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: is a class A network, but is reserved for use as a loopback address


Note 2: The 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

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, 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 where "168.212" identifies the network and "226.204" identifies the host on that network.

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 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.

( -

Class B: (16K-2) networks, each can have up to (64K-2) nodes

( -

Class C: (2M-2) networks, each can have up to 254 nodes.

( -

Class D: a multicast address.

( -

Class E: reserved for future use.

( –

Class-Based Subnet Masks

Class Class-Based Subnet Masks Class-Based Subnet Masks Usage Description
A /8 Very large networks, always sub netted
B /16 Large networks, typically sub netted
C /24 Small networks, the most common class
D /32 Multicasting group addresses (no hosts)
E Undefined Undefined 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 with 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, 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
/27 1/8th of a Class C 32 hosts
/26 1/4th of a Class C 64 hosts
/25 1/2 of a Class C 128 hosts
/24 1 Class C 256 hosts
/23 2 Class C 512 hosts
/22 4 Class C 1,024 hosts
/21 8 Class C 2,048 hosts
/20 16 Class C 4,096 hosts
/19 32 Class C 8,192 hosts
/18 64 Class C 16,384 hosts
/17 128 Class C 32,768 hosts
/16 256 Class C

(= 1 Class B)

65,536 hosts
/15 512 Class C 131,072 hosts
/14 1,024 Class C 262,144 hosts
/13 2,048 Class C 524,288 hosts

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 telephone number.

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.

User Impacts

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 support CIDR).

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 could take 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 if you 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

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