Curs PC si internet cap 10.6 Basics of Subnetting

10.6 Basics of Subnetting 10.6.1 Classical IP Addressing Instructor Note The purpose of this target indicator is to show the wastefulness of classical, classful, non-subnetted IP addressing. The problem may be phrased as follows. A certain amount of large address blocks -- 127 class As -- were created, with over 16 million hosts per network. Few if any of these large address blocks use all 16 million host numbers -- wasting IP addresses. A different, fairly large number, of medium sized address blocks -- over 65,000 class B addresses -- were created, with 65,000 hosts per network. This is still a large amount of hosts per network number -- again too many, wasting many hosts per network number.The largest amount of addresses -- the over 16 million class Cs -- only have 256 hosts per network number -- which is often too FEW hosts per network. So the division of networks into the sizes of classes A, B, and C, none of which is of optimum size for network administration, can be very wasteful in the assignment of hierarchical IP addresses. These inconvenient sizes for address classes are a remnant of an earlier day in the Internet's history, when it seemed unimaginable that any of the address classes would be almost completely assigned. But the proliferation of networks and hosts has made these classes limiting, and in advanced networking courses various ways of dealing with the consumption of addresses are taught (for example, VLSM (variable length subnet masking), private networks and network address translation, and IP version 6). Network administrators sometimes need to divide networks, particularly large networks, into smaller networks, called subnetworks, in order to provide addressing flexibility. Most of the time subnetworks are simply referred to as subnets. Similar to the host number portion of Class A, Class B, and Class C addresses, subnet addresses are assigned locally, usually by the network administrator. Also, like other IP addresses , each subnet address is unique. Web Links What's a Netmask? 10.6 Basics of Subnetting 10.6.2 Subnetwork Instructor Note The purpose of this target indicator is to introduce the abstract but vitally important topic of subnetting. Emphasize that we desire to give network administrators more flexibility, so we will allow them to extend the network number by a certain number of bits. Of course, this extension of the network number comes at the expense of the number of host bits. But this is not really harmful in the case of class A and class B addresses, which tend to have blocks of host addresses that are too large. The terminology is often that subnet bits are "borrowed" or "stolen"-- it is important to emphasize that the bits are being re-purposed. The notion of subnet mask is introduced -- the mask allows decoding of the subnetted network number. Without a subnet mask, the subnetwork number cannot be used to route data. Subnet addresses include the Class A, Class B, or Class C network portion, plus a subnet field and a host field. The subnet field and the host field are created from the original host portion for the entire network. The ability to decide how to divide the original host portion into the new subnet and host fields provides addressing flexibility for the network administrator. To create a subnet address, a network administrator borrows bits from the original host portion and designates them as the subnet field. Figures and illustrate the hierarchical nature of subnet addresses. To create a subnet address, a network administrator borrows bits from the host field and designates them as the subnet field. The minimum number of bits that can be borrowed is 2. If you were to borrow only 1 bit, to create a subnet, then you would only have a network number - the .0 network - and the broadcast number - the .1 network. The maximum number of bits that can be borrowed can be any number that leaves at least 2 bits remaining, for the host number. In this example of a Class C IP Address, bits from the host field for the subnet field have been borrowed. Web Links IP Address Subnetting Tutorial 10.6 Basics of Subnetting 10.6.3 Purpose for subnetting Instructor Note Besides the wastefulness of classical IP addressing and the improved efficiency of subnetting networks, there is another reason for using them. Smaller networks -- and remember, subnets are fully addressed networks to the "outside" world -- makes for smaller broadcast domains, an important consideration in network design. A primary reason for using subnets is to reduce the size of a broadcast domain. Broadcasts are sent to all hosts on a network or subnetwork. When broadcast traffic begins to consume too much of the available bandwidth, network administrators may choose to reduce the size of the broadcast domain. 10.6 Basics of Subnetting 10.6.4 Subnet mask Instructor Note The purpose of this target indicator is to provide the details of subnet masks. The longer name for subnet mask is instructive -- "extended network prefix". The mask's ones show how far we are extending the network number (at the expense of the host numbers). The subnet mask (formal term: extended network prefix), tells the network devices which part of an address is the network field and which part is the host field. A subnet mask is 32 bits long and has 4 octets, just like an IP address. To determine the subnet mask for a particular subnetwork IP address follow these steps. (1) Express the subnetwork IP address in binary form. (2) Replace the network and subnet portion of the address with all 1s. (3) Replace the host portion of the address with all 0s. (4) As the last step convert the binary expression back to dotted-decimal notation.Note: The extended network prefix includes the class A, B, or C network number, plus the subnet field (or subnet number) that is being used to extend the routing information (which is otherwise just the network number). 10.6 Basics of Subnetting 10.6.5 Boolean operations: AND, OR, and NOT Instructor Note There are three fundamental operations in Boolean algebra. These three functions are crucial in the design of all digital circuits, and important in programming. These functions are often used in "Boolean searches" to use Internet search engines to narrow the range of hits for a search. In internetworking, the AND function is particularly important part of the routing process. Teach one-bit Boolean AND as similar to multiplication ( 0 AND 0 = 0, 0 AND 1 = 0, 1 AND 0 = 0, 1 AND 1 = 1); one-bit Boolean OR as similar to addition (0 OR 0 = 0, 0 OR 1 = 1, 1 OR 0 = 1, 1 OR 1 = 1); and one-bit Boolean NOT as simply inversion of the bit (NOT 0 = 1 and NOT 1 = 0). This is also a good time to review the different ways 1s and 0s are sometimes represented -- ones as TRUE, ON, SHORT CIRCUIT, +5 Volts and zeros as FALSE, OFF, OPEN Circuit, or 0 Volts. For multiple-bit binary numbers, (anything AND 1111 1111) yields (anything). The term "operations" in mathematics refers to rules that define how one number combines with other numbers. Decimal number operations include addition, subtraction, multiplication, and division. There are related, but different, operations for working with binary numbers. The basic Boolean operations are AND, OR, and NOT.AND is like multiplication OR is like addition NOT changes 1 to 0, and 0 to 1 Web Links How Boolean Logic Works 10.6 Basics of Subnetting 10.6.6 Performing the AND function Instructor Note There are two keys to getting these types of problems correct. First, the student must be able to perform decimal to binary conversions. Secondly, they must understand the AND operation. Neatness is encouraged (lining up the bits and performing the bit-wise AND). Tricks are encouraged, like the idea that when AND is involved, any mask bits with a 1 copy the network id bits and any mask bits with a 0 result in a zero in the answer. The lowest numbered address in an IP network is the network address (the network number plus 0 in the entire host field). This also applies to a subnet: the lowest numbered address is the address of the subnet. Lab Activity In this lab, you will learn the basics of IP subnet masks and their use with TCP/IP networks. In order to route a data packet, the router must first determine the destination network/subnet address by performing a logical AND using the destination host's IP address and the subnet mask. The result will be the network/subnet address. In the Figure, the router has received a packet for host 131.108.2.2 - it uses the AND operation to learn that this packet should be routed to subnet 131.108.2.0. The process of ANDing is explained in Lab 10.6.6.