Networking Basics

This page describes general network concepts that you must understand before you configure your ImageStream router. See the glossary linked to our Other Resources page for unfamiliar terms.

Network Addressing

ImageStream routers support packet routing using the IP protocol. The Internet Protocol (IP) is a packet-based protocol used to exchange data over computer networks. IP provides addressing and control information that allows data packets to be routed across networks.

IP Addressing

IP address descriptions are found in RFC 1166, Internet Numbers. The Network Working Group maintains and distributes the RFC documents. The American Registry for Internet Numbers (ARIN) assigns IP addresses and network numbers to Internet service providers (ISPs), who in turn provide to their customers a range of addresses appropriate to the number of host devices on their network. The sections that follow describe the various types of IP addresses, how addresses are given, and routing issues related to IP.

IP Address Notation

IP addresses are written in dotted decimal notation consisting of four numbers separated by dots (periods). Each number, written in decimal, represents an 8-bit octet (sometimes informally referred to as a byte) giving each number a range of 0 through 255, inclusive. When strung together, the four octets form the 32-bit IP address. Table 1 shows 32-bit values expressed as IP addresses. The largest possible value of a field in dotted decimal notation is 255, which represents an octet where all the bits are 1s.

IP Address Classes

IP addresses are generally divided into different classes of addresses based on the number of hosts and subnetworks required to support the hosts. As described in RFC 1166, IP addresses are 32-bit quantities divided into five classes. Each class has a different number of bits allocated to the network and host portions of the address. For this discussion, consider a network to be a collection of computers (hosts) that have the same network field values in their IP addresses. The concept of classes is being made obsolete by classless interdomain routing (CIDR).

Instead of dividing networks by class, CIDR groups them into address ranges. A network range consists of an IP address prefix and a netmask length. The address prefix specifies the high-order bits of the IP network address. The netmask length specifies the number of high-order bits in the prefix that an IP address must match to fall within the range indicated by the prefix.

For example, 192.168.42.x describes a Class C network with addresses ranging from through CIDR uses to describe the same range of addresses. RIP version 1 is an example of a protocol that uses address classes. OSPF and BGP-4 are examples of protocols that do not use address classes.

32-Bit Value Dotted Decimal Notation
Table 1 - IP Address Notation

Class A Addresses

The class A IP address format allocates the highest 8 bits to the network field and sets the highest-priority bit to 0 (zero). The remaining 24 bits form the host field. Only 126 class A networks can exist (0 is reserved, and 127 is used for loopback networks), but each class A network can have almost 17 million hosts. No new class A networks can be assigned at this time.

For example:

          __ _________
          /      \
         /        \
      Network      \
      address    Host address

Class B Addresses

The class B IP address format allocates the highest 16 bits to the network field and sets the two highest-order bits to 1 and 0, providing a range from 128 through 191, inclusive. The remaining 16 bits form the host field. More than 16,000 class B networks can exist, and each class B network can have up to 65,534 hosts.

For example:

          _____ _______
            /      \
           /        \
       Network       \
       address    Host address

Class C Addresses

The class C IP address format allocates the highest 24 bits to the network field and sets the three highest-order bits to 1, 1, and 0, providing a range from 192 through 223, inclusive. The remaining 8 bits form the host field. More than two million class C networks can exist, and each class C network can have up to 254 hosts.

For example:

          ________ ___
             /      \
            /        \
       Network        \
       address    Host address

Class D Addresses

The class D IP address format was designed for multicast groups, as discussed in RFC 988. In class D addresses, the 4 highest-order bits are set to 1, 1, 1, and 0, providing a range from 224 through 239, inclusive. Class D addresses are currently used primarily for the multicast backbone (MBONE) of the Internet.

Class E Addresses

The class E IP address is reserved for future use. In class E addresses, the 4 highest-order bits are set to 1, 1, 1, and 1. Routers currently ignore class E IP addresses.

Reserved IP Addresses

Some IP addresses are reserved for special uses and cannot be used for host addresses. Table 2 lists ranges of IP addresses and shows which addresses are reserved, which are available to be assigned, and which are for broadcast.

ClassIP AddressStatus
A0.0.0.0Reserved through networks on the local host
B through
C through
D224.0.0.0 through group addresses
E240.0.0.0 through
Table 2 - Reserved and Available IP Addresses

Private IP Networks

RFC 1597 reserves three IP network addresses for private networks. The addresses,, and can be used by anyone for setting up their own internal IP networks.

IP Address Conventions

If the bits in the host portion of an address are all 0, that address refers to the network specified in the network portion of the address. For example, the class C address refers to a particular network. Historically, this address was used as a broadcast.

The standard for broadcast is high, which uses all 1s in the host portion (for example,; however, many networks still use all 0s. The ImageStream router can be configured either way and should be set to match the other systems on your network. Note: Do not assign an IP address with all 0s or all 1s in the host portion of the address to a host on the network, because these are reserved as broadcast addresses. With CIDR, networks are specified with an IP prefix and netmask length--for example,,, or


A netmask is a four-octet number that identifies either a supernetwork (supernet) or a subnetwork (subnet). A netmask that designates a subnet is called a subnet mask.

Using Subnet Masks to Create IP Subnets

Subnet masks are used to divide networks into smaller, more manageable groups of hosts known as subnets. Subnetting is a scheme for imposing a hierarchy on hosts on a single physical network. The usual practice is to use the first few bits in the host portion of the network address for a subnet field. RFC 950, Internet Standard Subnetting Procedure, describes subnetting.

A subnet mask identifies the subnet field of a network address. This mask is a 32-bit number written in dotted decimal notation with all 1s (ones) in the network and subnet portions of the address, and all 0s (zeros) in the host portion. This scheme allows for the identification of the host portion of any address on the network. Table 3 shows the subnet masks you can use to divide a class C network into subnets.

Length (Subnet Bits)Number of SubnetsNumber of Hosts per SubnetHexadecimal Subnet MaskDotted Decimal Subnet Mask
Table 3 - Subnet Masks for a Class C Network
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