Network Protocols Internet Protocol IP Addresses mostly about IPv4 IPv6 is later TDC375 Autumn 2009 10 John Kristoff DePaul University 1 Why have addreses at layer 3 Aren t there already globally unique addresses at L2 If not should there be TDC375 Autumn 2009 10 John Kristoff DePaul University 2 IP address semantics Virtual not specific to a hardware device 32 bit fixed address length IPv4 Unique address for each interface typically Registry or upstream ISP assigns net bits prefix Local admin assigns subnet and host bits suffix Usually written in dotted decimal dotted quad e g 140 192 5 1 TDC375 Autumn 2009 10 John Kristoff DePaul University 3 IP address notation diagram courtesy of http www netbook cs purdue edu TDC375 Autumn 2009 10 John Kristoff DePaul University 4 Special IP addresses diagram courtesy of http www netbook cs purdue edu TDC375 Autumn 2009 10 John Kristoff DePaul University 5 Classful IP addressing diagram courtesy of http www netbook cs purdue edu TDC375 Autumn 2009 10 John Kristoff DePaul University 6 Classful address sizes diagram courtesy of http www netbook cs purdue edu TDC375 Autumn 2009 10 John Kristoff DePaul University 7 Example IP network diagram courtesy of http www netbook cs purdue edu TDC375 Autumn 2009 10 John Kristoff DePaul University 8 Classful addressing limitations Internet growth and address depletion Route table size potentially lots of class C nets Misappropriation of addresses Lack of support for varying sized networks Class B is often too big Class C often too small TDC375 Autumn 2009 10 John Kristoff DePaul University 9 IP addressing solutions Subnetting Supernetting Classless interdomain routing CIDR Variable length subnet masks VLSM Temporary addresses e g BOOTP DHCP NATs with port address translation blech TDC375 Autumn 2009 10 John Kristoff DePaul University 10 Subnetting TDC375 Autumn 2009 10 John Kristoff DePaul University 11 Subnet masks The bit length of the prefix or the network bits No more A B or C class addresses important Use of the slash notation to represent addresses 140 192 5 1 with mask of 255 255 255 128 is 140 192 5 1 25 A 25 mask in binary is 11111111 11111111 11111111 10000000 TDC375 Autumn 2009 10 John Kristoff DePaul University 12 Subnet masks example Given 140 192 50 8 20 what is the subnet mask in dotted decimal notation directed broadcast address in dotted quad total number of hosts that can be addressed TDC375 Autumn 2009 10 John Kristoff DePaul University 13 Supernetting Combine smaller address blocks into an aggregate If class B is too big and class C is too small Combine 199 63 0 0 24 to 199 63 15 0 24 To form 199 63 0 0 20 TDC375 Autumn 2009 10 John Kristoff DePaul University 14 CIDR classless inter domain routing Routers announce prefixes Maintain and announce millions of 24 s or Aggregate thank you supernetting So instead of adding multiple class C blocks Check out The Internet CIDR report http www cidr report org TDC375 Autumn 2009 10 John Kristoff DePaul University 15 CIDR example Given an ISP that announces 64 5 0 0 20 64 5 16 0 20 192 0 2 0 25 192 0 2 192 26 192 0 2 128 26 What is the least number of CIDR announcements that can be made for this ISP Why might address blocks not be aggregated TDC375 Autumn 2009 10 John Kristoff DePaul University 16 VLSM variable length subnet masks Many subnet sizes in an autonomous system AS Allows for efficient use of address space Can be used to build an internal hierarchy External view of the AS does not change An AS may be allocated 140 192 0 0 16 but internally may use 140 192 0 0 17 140 192 128 0 24 140 192 129 0 25 and so on TDC375 Autumn 2009 10 John Kristoff DePaul University 17 VLSM example Given an assignment of 140 192 0 0 16 create an addressing strategy to support 6 satellite offices and 1 large headquarter site 6000 total hosts on all combined networks headquarters needs about 50 of all addresses satellite offices need 200 to 700 addresses overall growth per year is 500 hosts TDC375 Autumn 2009 10 John Kristoff DePaul University 18 Obtaining IP addresses IANA has global authority for assignment RIRs delegate to ISPs and large nets ISPs assign addresses to end users and small nets RFC 1918 defines private address blocks NOT globally unique NOT for hosts attached directly to public Internet 10 0 0 0 8 172 16 0 0 12 and 192 168 0 0 16 TDC375 Autumn 2009 10 John Kristoff DePaul University 19 IP address types Unicast one to one Multicast one to many receivers join listen to group destination address Broadcast one to all source addresses should always be unicast special case of multicast usually unnecessary Anycast one to one of many usually one to nearest often used for reliability TDC375 Autumn 2009 10 John Kristoff DePaul University 20 NAT network address translation Originally a solution to a shortage problem Became a security monkey s best practice It can help alleviate renumbering problems But if the net is that big get your own allocation NAT back slaps the e2e argument in the face What has been a big motivating factor for Ipv6 Is NAT for IPv6 for rational IETF try as you might to standardize it away TDC375 Autumn 2009 10 John Kristoff DePaul University 21 Final thoughts IP addressing is a pain wait til you see IPv6 IP addresses today are both a who and a where IP addresses make for poor trust relationships Private addresses and NATs blech Get a real net TDC375 Autumn 2009 10 John Kristoff DePaul University 22
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