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.6.3.12.Systems on net 128.6.3 will list 128.6.3.12 asthe gateway for many off-campus networks.However systems on net 128.6.4 willlist 128.6.4.1 as the gateway to those same off-campus networks.128.6.4.1 isthe gateway between networks 128.6.4 and128.6.3, so it is the first step in getting to JvNC.When a computer wants to send a datagram, it first checks to see if thedestination address is on the system's own local network.If so, the datagramcan be sent directly.Otherwise, the system expects tofind an entry for the network that the destination address is on.The datagramis sent to the gateway listed in that entry.This table can get quite big.Forexample, the Internet now includes several hundredindividual networks.Thus various strategies have been developed to reduce thesize of the routing table.One strategy is to depend upon "default routes".Often, there is only one gateway out of a network.This gateway might connect alocal Ethernet to a campus-wide backbone network.In that case, we don't needto have a separate entry for every network in the world.We simply define thatgateway as a "default".When no specific route is found for a datagram, thedatagram is sent to the default gateway.A default gateway can even be usedwhen there are several gateways on a network.There are provisions for gatewaysto send a message saying "I'm not the best gateway -- use this one instead."(The message is sent via ICMP.See RFC 792.) Most network software is designedto use these messages to add entries to their routing tables.Suppose network128.6.4 has two gateways, 128.6.4.59 and 128.6.4.1.128.6.4.59 leads to severalother internal Rutgers networks.128.6.4.1 leads indirectly to the NSFnet.Suppose we set 128.6.4.59 as a default gateway, and have no other routing tableentries.Now what happens when we need to send a datagram to MIT? MIT isnetwork 18.Since we have no entry for network 18, the datagram will be sent tothe default, 128.6.4.59.As it happens, this gateway is the wrong one.So itwill forward thedatagram to 128.6.4.1.But it will also send back an error saying in effect:"to get to network 18, use 128.6.4.1".Our software will then add an entry tothe routing table.Any future datagrams to MIT will then go directly to128.6.4.1.(The error message is sent using the ICMP protocol.The message typeis called "ICMP redirect.")Most IP experts recommend that individual computers should not try to keeptrack of the entire network.Instead, they should start with default gateways,and let the gateways tell them the routes, as justdescribed.However this doesn't say how the gateways should find out about theroutes.The gateways can't depend upon this strategy.They have to have fairlycomplete routing tables.For this, some sort ofrouting protocol is needed.A routing protocol is simply a technique for thegateways to find each other, and keep up to date about the best way to get toevery network.RFC 1009 contains a review ofgateway design and routing.However rip.doc is probably a better introductionto the subject.It contains some tutorial material, and a detailed descriptionof the most commonly-used routing protocol.[2.1.8] Subnets and BroadcastingDetails about Internet Addresses: Subnets and BroadcastingAs indicated earlier, Internet addresses are 32-bit numbers, normally writtenas 4 octets (in decimal), e.g.128.6.4.7.There are actually 3 different typesof address.The problem is that the address has toindicate both the network and the host within the network.It was felt thateventually there would be lots of networks.Many of them would be small, butprobably 24 bits would be needed to represent all the IPnetworks.It was also felt that some very big networks might need 24 bits torepresent all of their hosts.This would seem to lead to 48 bit addresses.Butthe designers really wanted to use 32 bitaddresses.So they adopted a kludge.The assumption is that most of the networks will be small.So they set up threedifferent ranges of address.Addresses beginning with 1 to 126 use only thefirst octet for the network number.The other three octets are available forthe host number
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