Network: IP address

IP addresses are broken into 4 octets (IPv4) separated by dots called dotted decimal notation. An octet is a byte consisting of 8 bits. The IPv4 addresses are in the following form:

192.168.10.1

There are two parts of an IP address:
● Network ID
● Host ID

The various classes of networks specify additional or fewer octets to designate the network ID versus the host ID.

When a network is set up, a netmask is also specified. The netmask determines the class of the network as shown below, except for CIDR. When the netmask is setup, it specifies some number of most significant bits with a 1's value and the rest have values of 0. The most significant part of the netmask with bits set to 1's specifies the network address, and the lower part of the address will specify the host address. When setting addresses on a network, remember there can be no host address of 0 (no host address bits set), and there can be no host address with all bits set.

Class A-E networks

The addressing scheme for class A through E networks is shown below. Note: We use the 'x' character here to denote don't care situations which includes all possible numbers at the location. It is many times used to denote networks.

Network Type Address Range Normal Netmask Comments

Class A 001.x.x.x to 126.x.x.x 255.0.0.0 For very large networks

Class B 128.1.x.x to 191.254.x.x 255.255.0.0 For medium size networks

Class C 192.0.1.x to 223.255.254.x 255.255.255.0 For small networks

Class D 224.x.x.x to 239.255.255.255 Used to support multicasting

Class E 240.x.x.x to 247.255.255.255

RFCs 1518 and 1519 define a system called Classless Inter-Domain Routing (CIDR) which is used to allocate IP addresses more efficiently. This may be used with subnet masks to establish networks rather than the class system shown above. A class C subnet may be 8 bits but using CIDR, it may be 12 bits.

There are some network addresses reserved for private use by the Internet Assigned Numbers Authority (IANA) which can be hidden behind a computer which uses IP masquerading to connect the private network to the internet. There are three sets of addresses reserved. These address are shown below:

● 10.x.x.x

● 172.16.x.x - 172.31.x.x

● 192.168.x.x

Other reserved or commonly used addresses:
● 127.0.0.1 - The loopback interface address. All 127.x.x.x addresses are used by the loopback

interface which copies data from the transmit buffer to the receive buffer of the NIC when used.

● 0.0.0.0 - This is reserved for hosts that don't know their address and use BOOTP or DHCP protocols to determine their addresses.

● 255 - The value of 255 is never used as an address for any part of the IP address. It is reserved for broadcast addressing. Please remember, this is exclusive of CIDR. When using CIDR, all bits of the address can never be all ones.

To further illustrate, a few examples of valid and invalid addresses are listed below:

1. Valid addresses:

❍ 10.1.0.1 through 10.1.0.254

❍ 10.0.0.1 through 10.0.0.254

❍ 10.0.1.1 through 10.0.1.254 2.

2.Invalid addresses:

❍ 10.1.0.0 - Host IP can't be 0.

❍ 10.1.0.255 - Host IP can't be 255.

❍ 10.123.255.4 - No network or subnet can have a value of 255.

❍ 0.12.16.89 - No Class A network can have an address of 0.

❍ 255.9.56.45 - No network address can be 255.

❍ 10.34.255.1 - No network address can be 255.

Network/Netmask specification

Sometimes you may see a network interface card (NIC) IP address specified in the following manner:

192.168.1.1/24

The first part indicates the IP address of the NIC which is "192.168.1.1" in this case. The second part "/24" indicates the netmask value meaning in this case that the first 24 bits of the netmask are set. This makes the netmask value 255.255.255.0. If the last part of the line above were "/16", the netmask would be 255.255.0.0.

Subnet masks

Subnetting is the process of breaking down a main class A, B, or C network into subnets for routing purposes. A subnet mask is the same basic thing as a netmask with the only real difference being that you are breaking a larger organizational network into smaller parts, and each smaller section will use a different set of address numbers. This will allow network packets to be routed between subnetworks. When doing subnetting, the number of bits in the subnet mask determine the number of available subnets. Two to the power of the number of bits minus two is the number of available subnets. When setting up subnets the following must be determined:

● Number of segments

● Hosts per segment

Subnetting provides the following advantages:

● Network traffic isolation - There is less network traffic on each subnet.

● Simplified Administration - Networks may be managed independently.

● Improved security - Subnets can isolate internal networks so they are not visible from

external networks.

A 14 bit subnet mask on a class B network only allows 2 node addresses for WAN links. A routing algorithm like OSPF or EIGRP must be used for this approach. These protocols allow the variable length subnet masks (VLSM). RIP and IGRP don't support this. Subnet mask information must be transmitted on the update packets for dynamic routing protocols for this to work. The router subnet mask is different than the WAN interface subnet mask.

One network ID is required by each of:

● Subnet

● WAN connection

One host ID is required by each of:

● Each NIC on each host.

● Each router interface.

Types of subnet masks:

● Default - Fits into a Class A, B, or C network category

● Custom - Used to break a default network such as a Class A, B, or C network into subnets.

IPv6

IPv6 is 128 bits. It has eight octet pairs, each with 16 bits and written in hexadecimal as follows:

2b63:1478:1ac5:37ef:4e8c:75df:14cd:93f2

Extension headers can be added to IPv6 for new features.

Supernetting

Supernetting is used to help make up for some of the shortage if IP addresses for the internet. It uses Classless Inter-Domain Routing (CIDR). If a business needs a specific number of IP addresses such as 1500, rather than allocating a class B set of addresses with the subnet mask of 255.255.0.0, a subnet mask of 255.255.248.0 may be allocated. Therefore the equivalent of eight class C addresses have been allocated. With supernetting, the value of 2 is not subtracted from the possible number of subnets since the router knows that these are contiguous networks. 8 times 254 = 2032.

What section of this document to read next

At this point the reader should have enough fundamental knowledge to grasp routing, so the reader may continue on or skip to the section entitled, "simple routing". The reader may at this time read all the sections in the "Functions" group of sections, then continue back at the section after this one where you left off.

Each machine in the network shown below, has one or more network cards. The part of the network that does the job of transporting and managing the data across the network is called TCP/IP which stands for Transmission Control Protocol (TCP) and Internet Protocol (IP). There are other alternative mechanisms for managing network traffic, but most, such as IPX/SPX for Netware, will not be described here in much detail. The IP layer requires a 4 (IPv4) or 6 (IPv6) byte address to be assigned to each network interface card on each computer. This can be done automatically using network software such as dynamic host configuration protocol (DHCP) or by manually entering static addresses into the computer.

Ports

The TCP layer requires what is called a port number to be assigned to each message. This way it can determine the type of service being provided. Please be aware here, that when we are talking about "ports" we are not talking about ports that are used for serial and parallel devices, or ports used for computer hardware control. These ports are merely reference numbers used to define a service. For instance, port 23 is used for telnet services, and HTTP uses port 80 for providing web browsing service. There is a group called the IANA (Internet Assigned Numbers Authority) that controls the assigning of ports for specific services. There are some ports that are assigned, some reserved and many unassigned which may be utilized by application programs. Port numbers are straight unsigned integer values which range up to a value of 65535.

Addresses

Addresses are used to locate computers. It works almost like a house address. There is a numbering system to help the mailman locate the proper house to deliver customer's mail to. Without an IP numbering system, it would not be possible to determine where network data packets should go.

IPv4, which means internet protocol version 4, is described here. Each IP address is denoted by what is called dotted decimal notation. This means there are four numbers, each separated by a dot. Each number represents a one byte value with a possible mathematical range of 0-255. Briefly, the first one or two bytes, depending on the class of network, generally will indicate the number of the network, the third byte indicates the number of the subnet, and the fourth number indicates the host number. This numbering scheme will vary depending on the network and the numbering method used such as Classless Inter-Domain Routing (CIDR) which is described later. The host number cannot be 0 or 255. None of the numbers can be 255 and the first number cannot be 0. This is because broadcasting is done with all bits set in some bytes. Broadcasting is a form of communication that all hosts on a network can read, and is normally used for performing various network queries. An address of all 0's is not used, because when a machine is booted that does not have a hardware address assigned, it provides 0.0.0.0 as its address until it receives its assignment. This would occur for machines that are remote booted or those that boot using the dynamic host configuration protocol (DHCP). The part of the IP address that defines the network is referred to as the network ID, and the latter part of the IP address that defines the host address is referred to as the host ID. IPv6 is an enhancement to the IPv4 standard due to the shortage of internet addresses. The dotted notation values are increased to 12 bit values rather than byte (8 bit) values. This increases the effective range of each possible decimal value to 4095. Of course the values of 0 and 4095 (all bits set) are generally reserved the same as with the IPv4
standard.

Network

Sunday, February 2, 2014

Network Addressing