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.Their reply will contain the fullyqualified network address, which the sender will record for future use.all bits 1Stands for all: all networks or all hosts.For example, the following means allhosts on network 128.2 (class B address):128.2.255.255This is called a directed broadcast address because it contains both a validand a broadcast.LoopbackThe class A network 127.0 is defined as the loopback network.Addresses from that network are assigned to interfaces that process datainside the local system and never access a physical network (loopbackinterfaces).2.1.2 IP SubnetsDue to the explosive growth of the Internet, the principle of assigned IP addressesbecame too inflexible to allow easy changes to local network configurations.Thosechanges might occur when:A new type of physical network is installed at a location.Growth of the number of hosts requires splitting the local network into two ormore separate networks.Growing distances require splitting a network into smaller networks, withgateways between them.To avoid having to request additional IP network addresses in these cases, theconcept of subnets was introduced.The assignment of subnets can be donelocally, as the whole network still appears to be one IP network to the outsideworld.The host number part of the IP address is subdivided again into a network numberand a host number.This second network is termed a subnetwork or subnet.Themain network now consists of a number of subnets and the IP address isinterpreted as:The combination of the subnet number and the host number is often termed thelocal address or the local part.Subnetting is implemented in a way that istransparent to remote networks.A host within a network that has subnets is awareof the subnetting but a host in a different network is not; it still regards the local partof the IP address as a host number.30 TCP/IP Tutorial and Technical OverviewThe division of the local part of the IP address into subnet number and hostnumber parts can be chosen freely by the local administrator; any bits in the localpart can be used to form the subnet.The division is done using a subnet maskwhich is a 32 bit number.Zero bits in the subnet mask indicate bit positionsascribed to the host number, and ones indicate bit positions ascribed to the subnetnumber.The bit positions in the subnet mask belonging to the network number areset to ones but are not used.Subnet masks are usually written in dotted decimalform, like IP addresses.The special treatment of all bits zero and all bits one applies to each of the threeparts of a subnetted IP address just as it does to both parts of an IP address thathas not been subnetted (see 2.1.1.2,  Special IP Addresses on page 30).Forexample, a subnetted Class B network, which has a 16-bit local part, could use oneof the following schemes:The first byte is the subnet number; the second byte is the host number.Thisgives us 28-2 (254 with the values 0 and 255 being reserved) possible subnets,each having up to 28-2 (254) hosts.The subnet mask is 255.255.255.The first 12 bits are used for the subnet number and the last four for the hostnumber.This gives us 212-2 (4094) possible subnets but only 24-2 (14) hostsper subnet.The subnet mask is 255.255.255.240.There are many other possibilities.In fact, the number of subnets and hostsand future requirements should be taken into consideration before defining asubnet.In the above example, for a subnetted Class B network, there are 16bits left for the subnet number and the host number fields.The administratorhas the choice of defining either a larger number of subnets with a smallnumber of hosts in each, or a smaller number of subnets with many hosts.While the administrator is completely free to assign the subnet part of the localaddress in any legal fashion, the objective is to assign a number of bits to thesubnet number and the remainder to the local address.Therefore, it is normal touse a contiguous block of bits at the beginning of the local address part for thesubnet number because this makes the addresses more readable.(This isparticularly true when the subnet occupies 8 or 16 bits.) With this approach, eitherof the subnet masks above are  good masks, but masks such as 255.255.252.252and 255.255.255.15 are not [ Pobierz całość w formacie PDF ]