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IP Addressing: Basics, Classes, and CIDR

Learn about IP addressing, including the basics of IP addresses, classful addressing (obsolete), and classless inter-domain routing (CIDR).

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IP Addressing: Basics, Classes, and CIDR

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  1. COMS W4995-1Lecture 3

  2. IP Addressing Today: • IP addressing • Data link protocols and ARP • Notes about lab

  3. IP Addressing • Addressing defines how addresses are allocated and the structure of addresses • IPv4 • Classful IP addresses (obsolete) • Classless inter-domain routing (CIDR) (RFC 854, current standard) • IP Version 6 addresses

  4. What is an IP Address? • Why Addresses? • End-to-end argument (principle) • Reading: http://web.mit.edu/Saltzer/www/publications/endtoend/endtoend.pdf • Keep it Simple, Stupid

  5. What is an IP Address? • An IP address is a unique global address for a network interface. • An IP address uniquely identifies a network location. • http://www.arin.net/whois • http://www.iana.org/ipaddress/ip-addresses.htm • Routers forwards a packet based on the destination address of the packet.

  6. IPv4 Addresses

  7. IP v.4 Addresses 32 bits 0x4 0x5 0x00 44 10 9d08 010 0000000000000 2 2 128 0x06 8bff 10 128.143.137.144 128.143.71.21

  8. IP v.4 Addressing • An IP address is often written in dotted decimal notation • Each byte is identified by a decimal number in the range [0..255]: 10000000 10001111 10001001 10010000 1st Byte = 128 2nd Byte = 143 3rd Byte = 137 4th Byte = 144 128.143.137.144

  9. Structure of an IP address • An IP address encodes both a network number (network prefix) and an interface number (host number). • network prefix identifies a network • the host number identifies a specific host (actually, interface on the network). 31 0 network prefix host number

  10. How long the network prefix is? • Before 1993: The network prefix is implicitly defined (class-based addressing) • After 1993: The network prefix is indicated by a netmask.

  11. Before 1993: Class-based addressing • The Internet address space was divided up into classes: • Class A:Network prefix is 8 bits long • Class B:Network prefix is 16 bits long • Class C:Network prefix is 24 bits long • Class D is multicast address • Class E is reserved

  12. Classful IP Adresses (Until 1993) • Each IP address contained a key which identifies the class: • Class A:IP address starts with “0” • Class B:IP address starts with “10” • Class C:IP address starts with “110” • Class D:IP address starts with “1110” • Class E:IP address starts wit “11110”

  13. The old way: Internet Address Classes

  14. The old way: Internet Address Classes

  15. The old way: Internet Address Classes

  16. Problems with Classful IP Addresses • Fast growing routing table size • Each router must have an entry for every network prefix • ~ 221 = 2,097,152 class C networks • In 1993, the size of routing tables started to outgrow the capacity of routers

  17. Other problems with classful addresses • Address depletion for large networks • Class A and Class B addresses were gone • How many class A/B network prefixes can there be? • Limited flexibility for network addresses: • Class A and B addresses are overkill (>64,000 addresses) • Class C address is insufficient (256 addresses)

  18. Classless Inter-domain routing (CIDR) 1993 • Full description RFC 1518 & 1519 • Network prefix is of variable length • Addresses are allocated hierarchically • Routers aggregate multiple address prefixes into one routing entry to minimize routing table size

  19. CIDR network prefix is variable length • A network mask specifies the number of bits used to identify a network in an IP address. • How? 144 16 128 59 Addr 10000000 10001111 10001001 10010000 255 255 0 255 Mask 11111111 11111111 1111111 00000000

  20. CIDR notation • CIDR notation of an IP address: • 128.143.137.144/24 • /24 is the prefix length. It states that the first 24 bits are the network prefix of the address (and the remaining 8 bits are available for specific host addresses) • CIDR notation can nicely express blocks of addresses • An address block [128.195.0.0, 128.195.255.255] can be represented by an address prefix 128.195.0.0/16 • How many addresses are there in a /x address block? • 2 (32-x)

  21. CIDR hierarchical address allocation 128.0.0.0/8 ISP • IP addresses are hierarchically allocated. • An ISP obtains an address block from a Regional Internet Registry • An ISP allocates a subdivision of the address block to an organization • An organization recursively allocates subdivision of its address block to its networks • A host in a network obtains an address within the address block assigned to the network 128.59.0.0/16 128.1.0.0/16 128.2.0.0/16 University 128.59.16.150 Foo.com Bar.com CS Library 128.59.16.0/24 128.59.44.0/24

  22. Hierarchical address allocation 128.59.16.[0 – 255] • ISP obtains an address block 128.0.0.0/8  [128.0.0.0, 128.255.255.255] • ISP allocates 128.59.0.0/16 ([128.59.0.0, 128.59.255.255]) to the university. • University allocates 128.59.16.0/24 ([128.59.16.0, 128.59.16.255]) to the CS department’s network • A host on the CS department’s network gets one IP address 128.59.16.150 128.59.16.150 128.59.0.0 – 128.59.255.255 128.0.0.0 - 128.255.255.255

  23. CIDR allows route aggregation I 128.0.0.0/8 ISP1 • ISP1 announces one address prefix 128.0.0.0./8 to ISP2 • ISP2 can use one routing entry to reach all networks connected to ISP1 You can reach 128.0.0.0/8 via ISP1 128.0.0.0/8 ISP3 ISP1 128.1.0.0/16 128.2.0.0/16 128.59.0.0/16 University Foo.com Bar.com CS Library

  24. CIDR summary • A network prefix is of variable length: a.b.c.d/x • Addresses are hierarchical allocated • Routers aggregate multiple address prefixes into one routing entry to minimize routing table size. • Security is still an issue • Secure Routing & Path validation

  25. What problems CIDR does not solve (I) 204.1.0.0/16 ISP1 128.0.0.0/8 ISP1 You can reach 128.0.0.0/8 And 204.1.0.0/16 via ISP1 • An multi-homing site still adds one entry into global routing tables ISP3 ISP1 ISP2 128.0.0.0/8 204.0.0.0/8 204.1.0.0/16 Mutil-home.com 204.1.0.0/16

  26. What problems CIDR does not solve (II) 204.1.0.0/16 ISP1 You can reach 128.0.0.0/8 And 204.1.0.0/16 via ISP1 • A site switches provider without renumbering still adds one entry into global routing tables ISP3 ISP1 ISP2 128.0.0.0/8 204.0.0.0/8 128.0.0.0/8 ISP1 204.1.0.0/16 Switched.com 204.1.0.0/16

  27. Global routing tables continue to grow Source: http://bgp.potaroo.net/as4637/

  28. Special IPv4 Addresses • Reserved or (by convention) special addresses: Loopback interfaces • all addresses 127.0.0.1-127.255.255.255 are reserved for loopback interfaces • Most systems use 127.0.0.1 as loopback address • loopback interface is associated with name “localhost” Broadcast address • Host number is all ones, e.g., 128.143.255.255 • Broadcast goes to all hosts on the network • Often ignored due to security concerns • Test / Experimental addresses • 10.0.0.0 - 10.255.255.255 • 172.16.0.0 - 172.31.255.255 • 192.168.0.0 - 192.168.255.255 • Convention (but not a reserved address) Default gateway has host number set to ‘1’, e.g., 128.195.4.1

  29. Special IPv4 Addresses (RFC 3330)

  30. IP Addressing (Summary) • Addressing defines how addresses are allocated and the structure of addresses • IPv4 • Classful IP addresses (obsolete) • Classless inter-domain routing (CIDR) (current standard) • IP Version 6 addresses

  31. IPv6 - IP Version 6 • IP Version 6 • Designed to be the successor to the currently used IPv4 • Specification completed in 1994 • Makes improvements to IPv4 (no revolutionary changes) • One (not the only !) feature of IPv6 is a significant increase in of the IP address to 128 bits (16 bytes) • IPv6 will solve – for the foreseeable future – the problems with IP addressing • 1024 addresses per square inch on the surface of the Earth.

  32. IPv6 Header

  33. Notation of IPv6 addresses • Convention: The 128-bit IPv6 address is written as eight 16-bit integers (using hexadecimal digits for each integer) CEDF:BP76:3245:4464:FACE:2E50:3025:DF12 • Short notation: Abbreviations of leading zeroes: CEDF:BP76:0000:0000:009E:0000:3025:DF12  CEDF:BP76:0:0:9E :0:3025:DF12 “:0000:0000:0000” can be written as “::” CEDF:BP76:0:0:FACE:0:3025:DF12  CEDF:BP76::FACE:0:3025:DF12

  34. IPv4 address in IPv6 • IPv6 addresses derived from IPv4 addresses have 96 leading zero bits. • Convention allows to use IPv4 notation for the last 32 bits. ::80:8F:89:90  ::128.143.137.144

  35. IPv6 vs. IPv4: Address Comparison • IPv4has a maximum of 232 4 billion addresses • IPv6 has a maximum of 2128 = (232)4  4 billion x 4 billion x 4 billion x 4 billion addresses Is IPv6 widely deployed?

  36. Data Link Layer • The main tasks of the data link layer are: • Transfer data from the network layer of one machine to the network layer of another machine • Convert the raw bit stream of the physical layer into groups of bits (“frames”)

  37. TCP/IP Protocol Stack • The TCP/IP protocol stack runs on top of multiple data link layers. • Two data link layer technologies • Broadcast • Point-to-Point

  38. Two types of networks at the data link layer • Broadcast Networks: All stations share a single communication channel • Point-to-Point Networks: Pairs of hosts (or routers) are directly connected • Typically, local area networks (LANs) are broadcast and wide area networks (WANs) are point-to-point

  39. Local Area Networks • Bus LAN • Ring LAN • Local area networks (LANs) connect computers within a building or a enterprise network • Almost all LANs are broadcast networks • Typical topologies of LANs are bus or ring or star • We will work with Ethernet LANs. Ethernet has a bus or star topology.

  40. MAC and LLC • In any broadcast network, the stations must ensure that only one station transmits at a time on the shared communication channel • The protocol that determines who can transmit on a broadcast channel are called Medium Access Control (MAC) protocol • The MAC protocol are implemented in the MAC sublayer which is the lower sublayer of the data link layer • The higher portion of the data link layer is often called Logical Link Control (LLC)

  41. IEEE 802 Standards • IEEE 802 is a family of standards for LANs, which defines an LLC and several MAC sublayers Higher layer issues LLC CSMA/CS Token bus Token ring Wireless lan

  42. Ethernet • Speed: 10Mbps -10 Gbps • Standard: 802.3, Ethernet II (DIX) • Most popular physical layers for Ethernet: • 10Base5 Thick Ethernet: 10 Mbps coax cable • 10Base2 Thin Ethernet: 10 Mbps coax cable • 10Base-T 10 Mbps Twisted Pair • 100Base-TX100 Mbps over Category 5 twisted pair • 100Base-FX100 Mbps over Fiber Optics • 1000Base-FX 1Gbps over Fiber Optics • 10000Base-FX 1Gbps over Fiber Optics (for wide area links)

  43. Bus Topology • 10Base5 and 10xBase2 Ethernets has a bus topology

  44. Star Topology • Starting with 10Base-T, stations are connected to a hub in a star configuration

  45. Ethernet Hubs vs. Ethernet Switches • An Ethernet switch is a packet switch for Ethernet frames • Buffering of frames prevents collisions. • Each port is isolated and builds its own collision domain • An Ethernet Hub does not perform buffering: • Collisions occur if two frames arrive at the same time. Hub Switch

  46. Ethernet and IEEE 802.3: Any Difference? • There are two types of Ethernet frames in use, with subtle differences: • “Ethernet” (Ethernet II, DIX (Digital-Intel-Xerox) • An industry standards from 1982 that is based on the first implementation of CSMA/CD by Xerox. • Predominant version of CSMA/CD in the US. • 802.3: • IEEE’s version of CSMA/CD from 1985. • Interoperates with 802.2 (LLC) as higher layer. • Difference for our purposes: Ethernet and 802.3 use different methods to encapsulate an IP datagram.

  47. Ethernet II, DIX Encapsulation (RFC 894)

  48. IEEE 802.2/802.3 Encapsulation (RFC 1042)

  49. Point-to-Point (serial) links • Many data link connections are point-to-point serial links: • Dial-in or DSL access connects hosts to access routers • Routers are connected by high-speed point-to-point links • Here, IP hosts and routers are connected by a serial cable • Data link layer protocols for point-to-point links are simple: • Main role is encapsulation of IP datagrams • No media access control needed

  50. Data Link Protocols for Point-to-Point links • SLIP (Serial Line IP) • First protocol for sending IP datagrams over dial-up links (from 1988) • Encapsulation, not much else • PPP (Point-to-Point Protocol): • Successor to SLIP (1992), with added functionality • Used for dial-in and for high-speed routers • HDLC (High-level Data Link Control) : • Widely used and influential standard (1979) • Default protocol for serial links on Cisco routers • Actually, PPP is based on a variant of HDLC

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