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Q and A, Ch. 21

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Q and A, Ch. 21
CS332,Fall 2017Victor Norman
Reason for IP addresses
Q: I'm still not super clear on the need for IPaddresses. I think it'sfor: 1. provide unique address for each machine on the internet;A: Yes2.tosend packet regardless the physics of the hardware(?);A: Yes3.identifythe type of network a host or the destination machine ison.A: No.
Role of IP address in routing
Q: When a packet is sent across networks, isthe IPaddress used to carry the packet all the way to the destination? or does itcarrythe packet to the correct LAN and the MAC address takes care of the rest?A: All the way. The destination IP address does not change. When it gets to the last router, the router sends the packet to the destination with the destinations MAC address in Ethernet header.
Dotted-decimal notation
Q: I still don't understand dotted decimal. Are we supposed to know how to convert from 32 bit to dotted decimal? If so how do I even begin to do that?A: An IP address is 4 bytes = 32 bits. One byte is a decimal number from 0 to 255. So, you convert each byte of the address to decimal and put a . between each part.
IP Address Parts
Every IP address has two (or 3) parts:Network part: uniquely identifies the network universally.Host part: uniquely identifies the host on the network.Why is this done?Routing isdone on the network part of an IP address.For scalability.Part of the host part can be used as a subnet part,within an organization.
ClassfulIP Addressing
(the old way)Figure 21.14 kinds of addressesDidn’t have to specify the mask, because you could figure it out from the address.Exercise: what class does belong to?
Address masks
Q: Can you explain address masks a bit more?A: An address mask indicates with 1 bits the part of an address that is the network part.The 1 bits are all consecutive and at the “left-most” part of the address.Can be shown as /nfirst n bits are 1swhich means the first n bits are the network part.E.g., Class C address 24-bit network part   /24NOTE: masks only needed inclassfuladdressing to indicate the subnet part.
Routers and masks
Q:Doesa router take in an IP address, mask it, then send it on to another network with the masked address?A: Masks are used in routing, but just to figure out what the network part of an address is. Masks are NOT sent in each IP header – onlysrcanddestIP addresses are there.
How to tell if a machine is on a network?
A machine with addressaddris on networkNwithmaskMifaddr& M == NN is 32-bitnetworkaddress (host part all 0s)& is binary ANDCould read this: if the network part ofaddrequals the networkN.Exercise: is on network 153.106.128 / 24 ?How about 153.106.128 / 17?Q: does every interface on a LAN have to have the same mask?
Subnet Addressing
Withclassfuladdressing, if you got a class B address, you had 1 LAN with 2^16 hosts on it.What if you wanted 2 LANs? (Why would you?)Internally in your network, you could subdivide your LAN into multiple LANs (orsubnets) using subnetting.“Borrow” some bits of the host part to indicate the subnet.1 bit 2 LANs; 2 bits  4 LANs; etc.Now, you have toroutebetween LANs in your organization.Now, every machine in your network has to know thenetmaskfor the network  so that it can figure out if another IP address is on its LAN or on another LAN.
Exercise, LLC, needs IP addresses for 500 computers (and we still live in aclassfuladdressing world)What class of IP addresses should they request?They want 3 subnets, for privacy/security:30 machines on one subnet,50 machines on another,420 machines on the last.What subnetting scheme should they use?How does “the world”’snetwork? I.e., what mask does “the world” use?
Classless Addressing
What is the problem with having only class A, B, or C address blocks?If you need, say, 256 hosts (which is > 254), you need class B, which means ~65,200 wasted IP addresses…What is the solution?Assign blocks of IP addresses to organizations based on any number of bits in the mask. (instead of just 8, 16, or 24 bits)e.g., 28 bit mask 2^4 – 2 hosts = 14 hosts.e.g., 23 bit mask  2 ^ 9 – 2 hosts = 510 hosts.Means many 23-bit IP address blocks can be given out to many companies, instead of 1 16-bit address block.Means far fewer wasted IP addresses and far more # of networks.
Tiny Corp wants a network with max 14 hosts.Network provider gives them 3 bytes have to be 154.100.1, top half of 4thbyte has to be all 0s. Hosts in the network will be from – is (all host bits are 1s).Joe’s Pizza asks for and gets network half of 4thbyte is 0001. Hosts are – network…16 networks with 154.100.1/28 can be allocated
Do classes matter anymore?
Q: In classless routing, do classes matter any more?A: Sort of. If you have what used to be a class B address (16-bit network part), then your network must have a mask of at least 16 bits.You can’t just have 8 bits, because there are 2^8 networks with the same top 8 bits.In other words, from looking at an address alone, you can tell the mask must be >= some #.
Transition to CIDR
Q: How did the transition to CIDR happen? What had to be done?A: Subnet masks were usedinternallywithin an organization already. With CIDR, masks needed to be used in ISPs (i.e., the Internet backbone) for proper routing.So, network masks needed to be advertised/distributed to all ISP routers along with the network numbers themselves.So, change in route propagation protocols (OSPF, RIP, BGPv4, etc.)
Watch out!
One problem with masks that aren’t 8, 16, or 24 bits is that IP addresses are still written8-bits . 8-bits . 8-bits . 8-bits.E.g., In CS at Calvin, we have machines on 153.106.116.* and 153.106.117.* that are on the same LAN… Hard to tell that from looking at the addresses alone.116 = 0111 0100, 117 = 0111 0101mask is /23 – 9 bits of the IP address are the host part.
Reuse of addresses
Q: Can IP addresses be reused within a network?A: Yes. IP addresses within an organization are usually assigned with DHCP. These addresses are “leased”, and can be taken back and given to another machine.
Address depletion, sales, etc.
Q: Can IP addresses be sold?A: I thought it was “no”, but it turns out it is “yes”.
Special addresses
Network address:allhostbits in the address are 0s.Used in routing tables, etc.Directed broadcast address:allhostbits in the address are 1s.when used as a destination address in a packet, it means the packet is routed to the destination network and then broadcast on it.Not done by routers today.
Special addresses (2)
Limited broadcast address32 1 bits.As a destination address, means send to all machines on the local Layer 2 network.Loopback address: – typically to test your local TCP/IP stack or to send some packet back to yourself so another task can process it.
“Joke shirt”
Multiple IP addresses on one machine?
Q: Is it possible for a host to have more than one IP address?A: Yes! If a host has multiple routinginterfacesthen it must have multiple IP addresses.A router/host has to decide how to get a packet to its destination: can the packet be sent directly to its destination or does it have to go through a router to be forwarded?So, each interface has to have an IP address (and know thenetmask) for the network it is on.
IP broadcast
Q: How much is IP broadcast used?A: Limited broadcast is used when you have to reach every/any machine on ”this” network. It is used for…DHCP requestARP request
Old slides
Universal Addressing Scheme
Q: Apart from the fact that arbitrary pairs of application programs can communicate without knowing the type of network hardware, what are the other needs for a universal addressing scheme?A: That’s about the only thing I can think of…
Dotted-decimal notation
Q: I still don't understand dotted decimal. Are we supposed to know how to convert from 32 bit to dotted decimal? If so how do I even begin to do that?A: An IP address is 4 bytes = 32 bits. One byte is a decimal number from 0 to 255. So, you convert each byte of the address to decimal and put a . between each part.
Non-computers have IP addresses?
Q: The chapter talks about computers having IP addresses but do other devices like printers have them as well?A: Yes! Anything that needs to talk on the network needs to have an IP address.
No hierarchical structure?
Q: Could the Internet have been designed with no hierarchical structure of IP addresses so none would be wasted?A: It could but it wouldn’t scale.Packets are routed based on their network part only.This allows one routing entry for hundreds or thousands of hosts. Without a hierarchy, you’d have to have an entry for every host.
Suffix not needed?
Q: If you apply a mask to an IP address, you only see the prefix right? Why? Don't you need to know the suffix as well?A: The mask is applied to the address in a host or router when deciding how to route the packet. It is not applied anywhere else. And, routing is based on the network an address belongs to, so to make this decision, the host/router doesn’t need to know the suffix.
Need for classless addressing
Q: Can you explain the need for classless addressing?A: IANA began to see that lots of addresses were being wasted. If your company needed 300 addresses, you couldn’t get a class C address (254 addresses max), so you got a class B, with 65534 addresses, most of which went unused. And, the number of class B networks was going down fast…
Lots of IP addresses available?
Q: Aren't there so many IP addresses available, that it would never matter if we were wasteful?A: No! The last set of addresses was given out by the IANA to a regional address registrar last year. We are now out of IP addresses.
More memory for CIDR?
Q: Did it use a lot more memory to start storing a 32 bit mask along with every 32 IP address?A: The question is, where is this extra memory required? The IP packet header didn’t change at all. Routing tables didn’t change either – because subnetting was available before CIDR. The only thing that changed is routing protocols, and they take more memory now, and the fact that ISPs can hand out non-/8, /16, and /24 blocks of addresses.
IPAddress Reuse
Q: Are IP addresses reused, and does a server store what IP addresses are being used and then know when one is no longer being used?A: IP addresses are re-used in a network that assigns them via DHCP. And, they could be reused in the Internet when one company’s address block is not used anymore.
Switching algorithm
in memory have a table that maps between port and mac address.receive frame on port n, with source macsmacanddestmacdmac.look upsmacin table. Add entry if not there. Update entry if there, and port is not n.ifdmacisbcast(all 1s), send to all ports, except n.look updmacin table: if not there, send frame to all ports except n. If there, send frame to only port associated withdmac.go through all entries in table and remove old ones.
Practice questions
Need a network with 2000 hosts. What class network would you get in the old days when we did classful networking? With classless addressing, your ISP would give you a network with what mask?
Practice questions (2)
For network many hosts can you have?what is the broadcast address?what is highest IP address?
Practice (3)
You get from your ISP.How many bits for host part?What is the limited broadcast address?You want 16 subnets, so what is your internal subnet mask?What is the lowest host IP address for subnet 3 (starting with 0)?





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Q and A, Ch. 21