Outline. IP Addressing, DHCP, and NATs. What is an IP Address? Addressing and Interfaces Subnets. How IP Addresses are Assigned.

Transcription

1 Outline What is an IP Address? Addressing and Interfaces Subnets How IP Addresses are Assigned Route Aggregation Host Addressing DHCP NAT

2 What is an IP Address? IP address for Millersville University is: xxxx To find your IP address or any IP address go to this link:

3 Addressing Interface: the boundary between the host or router and the physical link A router has multiple interfaces one for each of its links IP requires that each host and router interface has its own IP addresses An IP address is really associated with an interface rather than an end system For IPv4, each IP address is 32 bit long total of possible addresses Over 4 billion possibilities in addressing There is a format to which these addresses are chosen can t just randomly make up an address A portion of an interface s IP address will be determined by the subnet to which it is connected xxxxxxxx.xxxxxxxx.xxxxxxxx.xxxxxxxx Often represented as dotted-decimal notation: Binary notation:

4 IP Addressing and Interfaces router(s) interface(s) host(s) Take a careful look at the addresses Hub =

5 IP Addressing and Interfaces Subnet 1: interfaces IP address: xxx They have the same leftmost bits IP address assignment to subnet / 24 /24: subnet mask indicating that the leftmost bits of the bit quantity define the subnet address The rightmost bits are used to define the Subnet 1; e.g. Ethernet LAN Subnet Hub Subnet 2 Address Subnet 3 Address Subnet =

6 Subnets / /24 Generally, to determine the subnets. think of islands Each isolated network is called a subnet /24 Subnet mask: /24

7 Subnets IP Addressing, DHCP, and NATs In this example, each router has interfaces What subnets are present here? Broadcast links Point-to-point links

8 IP addressing: Classless InterDomain Routing (CIDR) There has to be some order and format in the assignment of addresses to subnets not random Internet s address assignment strategy: CIDR CIDR generalizes the notion of subnet addressing 32-bit IP address is divided into 2 parts: network and host addresses a.b.c.d/x where x indicates the # of bits in the first portion of the IP address - also referred to as the network prefix subnet part host part /23

9 IPv6 Addressing IPv6 addresses are denoted by eight groups of hexadecimal quartets separated by colons in between them. An example of a valid IPv6 address: 2001:cdba:0000:0000:0000:0000:3257:9652 Any 4-digit group of zeroes within an IPv6 address may be reduced to a single zero or altogether omitted. So, the following IPv6 addresses are similar and equally valid: 2001:cdba:0000:0000:0000:0000:3257: :cdba:0:0:0:0:3257:9652 (zero blocks can be substituted with a 0 ) 2001:cdba::3257:9652 (zero blocks can be omitted with :: ) IPv6 addresses can be: Unicast addressing Multicast addressing: a transmission to a group of interface cards on the network. Anycast addressing

10 IP Addresses: How are they Assigned? Generally, to obtain a block of ISP addresses for use within an organization a network administrator will contact its ISP The ISP will provide the paying organization with addresses from a larger block of addresses which have already been allocated to the ISP e.g. the ISP called CS395 itself has been allocated the address block of /20 The ISP can now divide its address block and assign organizations IP addresses The organizations, then use portions of their IP address for their The underlined part of the address represents the subnet CS395 ISP's block /20 Organization /23 Organization /23 Organization / Organization /23

11 IP Addresses: How are they Assigned? How does an ISP get block of addresses? ICANN: Internet Corporation for Assigned Names and Numbers IANA: Internet Assigned Numbers Authority is a department of ICANN responsible for allocating addresses to regional supporting registries which re-allocates addresses managing DNS root assigning domain names, resolving disputes Regional: ARIN, RIPE, APNIC, LACNIC : National Internet Registries Local Internet Registry

12 IP Addresses # (Cogent Communications) # (Millersville University) possibly / # (America Online) # (Telewest HSD Platform - UK) # (Verizon Internet Services) # (Verizon Internet Services) # (Comcast Cable) # (Energis- UK) # (Broadwing Communications) # (Telefonos del Noroeste S.A. de C.V. - Mexico)

13 Hierarchical Addressing: Route Aggregation The CS395 ISP can advertise to the outside world that it should be sent any datagrams whose first address bits match The rest of the Internet need not know about the addressing within/beyond this particular address block (the 8 organizations with their own subnet blocks) The ability to use a single prefix to advertise multiple networks is often referred to as address aggregation/ route aggregation/ route summarization Organization /23 hierarchical addressing allows efficient advertisement of routing information Organization /23 Organization /23 Organization 7.. CS395 ISP Send me anything with addresses beginning /20 Internet (Tier-1, Tier-2 ISPs) /23 ISPs-R-Us Send me anything with addresses beginning /16

14 Hierarchical Addressing: Route Aggregation What happens if CS395 ISP acquires ISPs-R-Us and then has Org 1 connect to the Internet through its new subsidiary? ISPs-R-Us owns the address block /16, but Org 1 s IP addresses are outside this address block!! Organization /23 ISPs-R-Us has a more specific route to Org 1 Organization /23 Organization /23 Organization /23.. CS395-ISP ISPs-R-Us Send me anything with addresses beginning /20 Send me anything with addresses beginning /16 Internet

15 Hosts Addressing When a host emits a datagram with destination address , the message is delivered to all hosts on the same subnet (broadcast). Once an organization has obtained a block of addresses, it can assign individual IP addresses to the host and router interfaces within its organization in 2 main ways: manual configuration a SA manually configures the IP addresses Dynamic Host Configuration Protocol (DHCP) allows a host to be allocated an IP address automatically, as well as to learn additional information, such as its subnet mask, the address of its first-hop router (default gateway) and the address of its local DNS server DHCP often referred to as a plug-and-play protocol popular with SAs widespread use in residential ISPs, wireless LANs - where hosts join and leave the network frequently: DHCP server will then assign temporary IP addresses very useful with the rise of mobile computing where a student will connect to a new subnet from dorm to classroom e.g. a residential ISP has 2,000 customers and no more than 400 of them are online at the same time does the ISP need 2,000 addresses? It needs a block of addresses. a.b.c.d/ )

16 DHCP Client-Server Scenario A DHCP server DHCP is a client-server protocol arriving DHCP client needs address in this network As hosts join and leave, the DHCP server allocates an arbitrary address from its current pool of available addresses Each time a host leaves, the address is returned to the pool How does the new host/ client know the address of the DHCP server? In fact how does it know the IP address of the network? A client/host arrives, wanting to join a network and wants an IP address for itself In the simplest case, there is a DHCP server for each subnet

17 DHCP Client-Server Scenario A DHCP Server DHCP Server DHCP Server DHCP protocol is a 4-step process DHCP discover message arriving client DHCP server: src : , 68 dest.: ,67 yiaddr: transaction ID: arriving DHCP client needs address in this network DHCP offer message src: , 67 dest: , 68 yiaddrr: transaction ID: 654 Lifetime: 3600 secs DHCP request message DHCP Server 3 src: , 68 dest:: , 67 yiaddrr: transaction ID: 655 Lifetime: 3600 secs Google: udp port dhcp DHCP relay agent time DHCP ACK message src: , 67 dest: , 68 yiaddrr: transaction ID: 655 Lifetime: 3600 secs

18 Network Address Translation (NAT) The address space /24 is one of the IP addresses reserved for home networks (small office, home office SOHO) It only has real meaning within that network NAT-enabled router looks like a normal router to the outside world rest of Internet local network (e.g., home network) / All datagrams leaving and entering local network have same single source NAT IP address: , different source port numbers

19 Network Address Translation (NAT) The NAT-enabled router is hiding the details of the home network from the outside world If all datagrams arriving at the NAT router from the WAN have the same destination IP address then how does the router know the internal host to which it should forward a given datagram suppose that both host and send a datagram to the LAN asking for the same Web page on the same Web server what happens

20 Network Address Translation (NAT) IP Addressing, DHCP, and NATs 2: NAT router changes datagram source addr from , 3345 to , 5001, updates table NAT translation table WAN side addr LAN side addr , , : host sends datagram to , 80 S: , 3345 D: , S: , 5001 D: , S: , 80 D: , : Reply arrives dest. address: , S: , 80 D: , : NAT router changes datagram dest addr from , 5001 to ,

21 Network Address Translation (NAT) Motivation range of addresses not needed from ISP: just one IP address for all devices can change addresses of devices in local network without notifying outside world can change ISP without changing addresses of devices in local network devices inside local net not explicitly addressable, visible by outside world (a security plus).

22 Objections to Network Address Translation (NAT) IETF Port numbers should be used for addressing processes, not hosts Hosts should communicate directly with each other without an IP-modifying node interfering router IPv6 solves the problem of potential shortages of IP addresses With which major application does NAT interfere?

23 Addendum: How it Used to be Classful Addressing Class # of Network Bits # of Host Bits Start End CIDR Equivalent Class A /8 Class B /16 Class C /24 Class D (multicast) NA Class E (reserved) NA 126 Class A networks that could include up to 16,777,214 hosts each 65,000 Class B networks that could include up to 65,534 hosts each over 2 million Class C networks that could include up to 254 hosts each (Some addresses are reserved for broadcast messages, etc.).

24 Example: Class C IP Addressing, DHCP, and NATs Network Bits Subnet Mask Number of Subnets Number of Hosts / / (0) 126 / (2) 62 / (6) 30 / (14) 14 / (30) 6 / (62) 2