Internet Addressing. Mr Nenad Krajnović

Transcription

1 Internet Addressing Mr Nenad Krajnović 1

2 What is an IP Address? 32-bit number, defined by the Internet Protocol (IP) (RFC 791). IP addresses must be unique within the network. One IP address may point to: one and only one destination on the Internet (unicast), or: one and only one group of destinations (multicast). IP addresses are 32-bit integers, oftenly written in 4-octet form: IP address distribution is coordinated in hierarchical manner. End users receive their address space from their ISP! 2

3 How many IP addresses exists? Since 32-bits are used for IP addresses, maximum number of IP addresses is: Complete address range can not be used for host addressing!

4 IP Address Structure IP address consists of two parts: Network prefix (m bits) Host id (32-m bits) 4

5 Network Address Addresses all hosts on a network segment: / Network prefix (m bits) Host id (32-m zeros) /24 - denotes prefix length (m = 24) 5

6 Subnet mask Subnet mask - a way of keeping information about prefix length Network prefix (m ones) Host id (32-m zeros) 6

7 Broadcast Address Used by IP to broadcast data to all hosts on a given subnet / Network prefix (m bits - network prefix) Host id (32-m ones) /24 - denotes the prefix length (m = 24) 7

8 IP Address Structure - a Review (Repetitio mater studiorum est) IP adress: Network address: /24 Subnet mask: Broadcast address: /24 8

9 Another Example... Prefix length does not have to be on octet boundaries: / Network prefix (m=26 bits) Host id (6 bits) /26 - denotes the prefix length (m = 26) 9

10 Another Example - Review IP adress: /26 Network address: /26 Subnet mask: Broadcast address: /26 10

11 Addressing in the LAN /

12 Addressing in the LAN (VLAN) VLAN /24 VLAN /24 VLAN /24 VLAN /23

13 Addressing in the WAN / / / / /

14 Why we are using network addresses? Routing table eth > eth > eth > eth > eth > eth > eth > eth > eth > eth > eth0 Routing table /26 -> eth0 eth / /

15 IP Address Classification Total available address space: We can classify addresses according to the following criteria: Classes: A, B, C, D, E class networks. Classes are now obsoleted! Usage: Public IP addresses (globally routeable, unique and non-reusable). Private IP addresses (routeable in private networks only). Special IP addresses (reserved, broadcast, multicast etc. networks). Validity: Provider aggregatable (valid until the ISP-User agreement is valid). Provider independent (valid until initial criteria for their assignment are met). 15

16 Network Prefix Length - m Prefix length may be determined: Automatically, depending on the address class - A, B, C, D, E (classful). Arbitary, depending on the ISP and customer network topology (CIDR). It defines the maximum number of addresses available to the user: 16 bits 24 bits 26 bits / addresses 256 / addresses 64 /26 64 addresses Assigned network prefix length may be expanded (subnetting) 16

17 Classful Addressing Today mostly obsoleted, used only by some routing protocols. Classful addressing was created in the past to ease allocations. IP adress space is divided to 5 IP adress classes - A, B, C, D i E: A B C D 0 remaining 31 bits remaining 30 bits remaining 29 bits remaining 28 bits E 1111 remaining 28 bits

18 Prefix Length and Classes Class IP range Start IP address m Subnet mask A 0/ B 128/ C 192/ D 224/ E 240/ Older routing protocols determined prefix length (m) by the class. Newer routing protocols explicitly transmit prefix length. 18

19 Classful Subnetting A customer is assigned an IP network on class boundaries: /16 (65534 hosts) The customer might expand the network prefix length, e.g.: /19 6 subnets, 8190 hosts/subnet /20 14 subnets, 4094 hosts/subnet / subnets, 254 hosts/subnet Subnet number of all 0 s and all 1 s is not allowed by default. To allow all 0 s and all 1 s to be subnet numbers, routers must be configured for that. 19

20 Why Classful Addressing? Routing protocols did not transport information about netmask to conserve necessary bandwidth on the links. With implicit definition of netmask, it was easier to implement routing protocols. Estimated number of hosts on the network was much less then available address space.

21 Drawbacks of Classful Addressing The customer must always be assigned a classful network. Classful addressing is a waste of address space: Formerly, users with more than 254 hosts had been assigned a B class. At the end of 1992, 70% of B class space was assigned. A route to each classful subnet must be specified separately: If an ISP had 254 customers, with addresses: , , separate route entries should be entered in the global routing table! These drawbacks led to a better solution - CIDR! 21

22 Classless Addressing (CIDR) Basis for classless inter-domain routing (CIDR). A customer is assigned an IP network on arbitary boundaries, e.g.: /19 (8192 hosts) The customer might expand the network prefix length, e.g.: /24 32 subnets, 254 hosts/subnet Subnet number of all 0 s and all 1 s is allowed by default. At the time of introduction of classless routing, it was necessary to additionally configure routers to support it. 22

23 Advantages of Classless Addressing The customer does not need to be assigned a classful network. Classful addressing saves address space: Formerly, users with more than 254 hosts had been assigned a B class. Now, they are assigned multiple C class networks (/23, /22, /21 etc.). Routes could be aggregated easily: If an ISP had 254 customers, with addresses: , , they will need a single entry in the routing table /16!!! Classless addressing (CIDR) introduced a better model of global address distribution process, defined by the RFC

24 FLSM vs VLSM Subnetting Fixed length subnet mask subnetting (FLSM): Typical in classful environments. Needed by older routing protocols, like RIPv1. All network segments should have the same network masks (prefixes). Not practical, when the network has a lot of segments, divided by routers. It may result in non-economical solutions (say, a /24 for a point-to-point link!). Variable length subnet mask subnetting (VLSM): Supported by major routing protocols today. Allows network segments, separated by routers, to have variable prefixes. Very practical, even in the point-to-point case (/30 assigned usually). Smaller percentage of address space loss (for reserved addresses). 24

25 Example Network Topology Access server PPP (unnumbered) Dial-in x 10 9 Ethernet 1 Repeater Total of 200 hosts Ethernet Embedded router 100 hosts PCs Ethernet 35 PCs PCs 5 40 PCs Router Router 30 PCs 20 PCs

26 Fixed-length Subnet Masks Repeater Ethernet Access server /24 Dial-in x /24 PPP (unnumbered) 50 PCs / Embedded router /24 Ethernet Ethernet /24 35 PCs /24 50 PCs /24 30 PCs /24 40 PCs /24 Router Router 20 PCs /24 26

27 Addressing Plan - FLSM i hosts Max addr Start address Prefix Subnet mask Broadcast / / / / / / / / / Network address and broadcast address can NOT be used for host addressing! 27

28 Variable-length Subnet Masks (VLSM) Repeater Ethernet Access server /24 Dial-in x /28 PPP (unnumbered) 50 PCs / Embedded router /24 Ethernet Ethernet /25 35 PCs /26 50 PCs /26 30 PCs /27 40 PCs /26 Router Router 20 PCs /27 28

29 Addressing Plan - VLSM i hosts Max addr Start address Prefix Subnet mask Broadcast / / / / / / / / / Network address and broadcast address can NOT be used for host addressing! 29

30 IP Address Distribution IANA Allocations RIPE NCC ARIN APNIC LACNIC AfriNIC ISP ISP ISP ISP ISP ISP ISP Assignments ISP 30

31 Internet Registries

32 Address Distribution - Example 193/8 IANA /19 ISP RIPE NCC / /18 ISP ISP / / /21 32

33 Information about address distribution? Every Internet Registry is maintaining public database about address assignment - RIPE - ARIN - LACNIC - APNIC - AfriNIC

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36 Address Categories Public IP numbers: Globally unique - one IP address points to one and only one destination. Non-reusable - once used, the same IP address must not be used elsewhere! Routable - theoretically visible from anywhere in the Internet. Need global coordinated allocation/assignment process (IANA, RIRs, ISPs ). Agreggatable - routes to /21 and i /21 give /20! Limited resource, like a radio-frequency spectrum! They need careful planning! Private IP numbers: Defined by the RFC 1918 (networks 10/8, /12 and /16). Basic application - intranets, networks behind firewalls or NATs or networks which won t connect to the Internet at all! They must not be routed on the public networks! Need coordination withing the private network where they are used. 36

37 Special IP Addresses Used internally by various networking protocols. They must not be used for host/network addressing. IP network m Abbreviated IP network usage /0 0/0 Default gateway /8 127/8 Loopback network /4 224/2 Multicast addresses (class D) /4 240/4 Class E (reserved by IANA) Special addresses are also: network address, subnet mask, broadcast address. 37

38 Address Validity (RFC 2050) Provider aggregatable (PA) address space: Assigned by the ISPs, for the need of their end users. Valid until the agreement between the ISP and the customer is valid. Upon provider change, the user must renumber their network! Provider guarantees global uniqueness and visibility of the assigned addresses. Most of assignments, being done today, are provider aggregatable. Provider independent (PI) address space: Address space, formerly assigned directly by InterNIC, RIPE NCC, APNIC. InterNIC, RIPE NCC, APNIC do not assign addresses to end users any more! The ISP reserves the right to refuse to route them, or to extra charge the cost for their routing. Necessary for multihomed networks! 38

39 Conclusion Addressing - Before and Now Before - classful addressing: End users received addresses from InterNIC, RIPE NCC, APNIC. A whole classful network address had to be assigned (say, C class). Fixed-length subnet masks in the user network. Avoidance of using zero subnets. Network prefix length derived directly from the address class. When changing the ISP, the user did not need to renumber the network! Now - CIDR: End users receive addresses from their ISPs. An arbitary network prefix can be assigned (say, /26 = ¼ C class). Variable-length subnet masks in the user network. Normal usage of zero subnets. Network prefix length information transmitted along with the netnum. When changing the ISP, the user has to renumber the network! 39