Naming and Addressing An Engineering Approach to Computer Networking

Outline Names and addresses Names and addresses Hierarchical naming Hierarchical naming Addressing Addressing Addressing in the telephone network Addressing in the telephone network Addressing in the Internet Addressing in the Internet ATM addresses ATM addresses Name resolution Name resolution Finding datalink layer addresses Finding datalink layer addresses

Names and addresses Names and addresses both uniquely identify a host (or an interface on the host) Names and addresses both uniquely identify a host (or an interface on the host) %nslookup %nslookup  Default Server: DUSK.CS.CORNELL.EDU  Address:  > underarm.com  Name: underarm.com  Address: Resolution: the process of determining an address from a name Resolution: the process of determining an address from a name

Why do we need both? Names are long and human understandable Names are long and human understandable  wastes space to carry them in packet headers  hard to parse Addresses are shorter and machine understandable Addresses are shorter and machine understandable  if fixed size, easy to carry in headers and parse Indirection Indirection  multiple names may point to same address  can move a machine and just update the resolution table

Figure Hierarchy concept in a telephone number

Hierarchical naming Goal: give a globally unique name to each host Goal: give a globally unique name to each host Naïve approach: ask other naming authorities before choosing a name Naïve approach: ask other naming authorities before choosing a name  doesn’t scale (why?)  not robust to network partitions Instead carve up name space (the set of all possible names) into mutually exclusive portions => hierarchy Instead carve up name space (the set of all possible names) into mutually exclusive portions => hierarchy

Hierarchy A wonderful thing! A wonderful thing!  scales arbitrarily  guarantees uniqueness  easy to understand Example: Internet names Example: Internet names  use Domain name system (DNS)  global authority (Network Solutions Inc.) assigns top level domains to naming authorities (e.g..edu,.net,.cz etc.)  naming authorities further carve up their space  all names in the same domain share a unique suffix

Addressing in the telephone network Telephone network has only addresses and no names (why?) Telephone network has only addresses and no names (why?) E.164 specifications E.164 specifications ITU assigns each country a unique country code ITU assigns each country a unique country code Naming authority in each country chooses unique area or city prefixes Naming authority in each country chooses unique area or city prefixes Telephone numbers are variable length Telephone numbers are variable length  this is OK since they are only used in call establishment Optimization to help dialing: Optimization to help dialing:  reserve part of the lower level name space to address top level domains  e.g. in US, no area code starts with 011, so 011 => international call => all other calls need fewer digits dialed

Figure Finding the class in binary notation

Figure Finding the address class

Example 3 Find the class of each address: 0 a b Solution See the procedure in Figure a.The first bit is 0; this is a class A address. b.The first 4 bits are 1s; this is a class E address.

Figure Finding the class in decimal notation

Example 4 Find the class of each address: a b c Solution a.The first byte is 227 (between 224 and 239); the class is D. b.The first byte is 252 (between 240 and 255); the class is E. c.The first byte is 134 (between 128 and 191); the class is B.

Figure Netid and hostid

Figure Blocks in class A

Millions of class A addresses are wasted. Note:

Figure Blocks in class B

Many class B addresses are wasted. Note:

The number of addresses in class C is smaller than the needs of most organizations. Note:

Figure Blocks in class C

Figure Network address

In classful addressing, the network address is the one that is assigned to the organization. Note:

Example 5 Given the address , find the network address. Solution The class is A. Only the first byte defines the netid. We can find the network address by replacing the hostid bytes ( ) with 0s. Therefore, the network address is

Example 6 Given the address , find the network address. Solution The class is B. The first 2 bytes defines the netid. We can find the network address by replacing the hostid bytes (17.85) with 0s. Therefore, the network address is

Example 7 Given the network address , find the class. Solution The class is A because the netid is only 1 byte.

A network address is different from a netid. A network address has both netid and hostid, with 0s for the hostid. Note:

Figure Sample internet

IP addresses are designed with two levels of hierarchy. Note:

Figure A network with two levels of hierarchy

Figure A network with three levels of hierarchy (subnetted)

Figure Addresses in a network with and without subnetting

Address evolution This scheme was too inflexible This scheme was too inflexible Three extensions Three extensions  subnetting  CIDR  dynamic host configuration

CIDR Scheme forced medium sized nets to choose class B addresses, which wasted space Scheme forced medium sized nets to choose class B addresses, which wasted space Address space exhaustion Address space exhaustion Solution Solution  allow ways to represent a set of class C addresses as a block, so that class C space can be used  use a CIDR mask  idea is very similar to subnet masks, except that all routers must agree to use it  subnet masks are not visible outside the network (why?)

CIDR (contd.)

Dynamic host configuration Allows a set of hosts to share a pool of IP addresses Allows a set of hosts to share a pool of IP addresses Dynamic Host Configuration Protocol (DHCP) Dynamic Host Configuration Protocol (DHCP) Newly booted computer broadcasts discover to subnet Newly booted computer broadcasts discover to subnet DHCP servers reply with offers of IP addresses DHCP servers reply with offers of IP addresses Host picks one and broadcasts a request to a particular server Host picks one and broadcasts a request to a particular server All other servers withdraw offers, and selected server sends an ack All other servers withdraw offers, and selected server sends an ack When done, host sends a release When done, host sends a release IP address has a lease which limits time it is valid IP address has a lease which limits time it is valid Server reuses IP addresses if their lease is over Server reuses IP addresses if their lease is over Similar technique used in Point-to-point protocol (PPP) Similar technique used in Point-to-point protocol (PPP)

DHCP With the rapid growth of TCP/IP (Transmission Control Protocol/Internet Protocol, the common transmission protocol for communicating over the Internet) networks, tools are needed to automate administrative functions in managing large TCP/IP networks. With the rapid growth of TCP/IP (Transmission Control Protocol/Internet Protocol, the common transmission protocol for communicating over the Internet) networks, tools are needed to automate administrative functions in managing large TCP/IP networks. The Dynamic Host Configuration Protocol (DHCP) is a set of rules for dynamically allocating IP addresses and configuration options to workstations on a network. The Dynamic Host Configuration Protocol (DHCP) is a set of rules for dynamically allocating IP addresses and configuration options to workstations on a network. An IP (Internet Protocol) address is a 32-bit binary number written as four decimal numbers separated by periods that is used to uniquely identify a workstation on the Internet. An IP (Internet Protocol) address is a 32-bit binary number written as four decimal numbers separated by periods that is used to uniquely identify a workstation on the Internet. An Internet address (like or ) is analogous to a telephone number. An Internet address (like or ) is analogous to a telephone number. While the telephone network directs calls to you by using your telephone number, the Internet network directs data to you by using your IP number. While the telephone network directs calls to you by using your telephone number, the Internet network directs data to you by using your IP number. This number can be statically (or manually) assigned by the administrator for a network workstation or assigned to it dynamically by a central server. This number can be statically (or manually) assigned by the administrator for a network workstation or assigned to it dynamically by a central server.

Who supports this protocol ? Most Network Operating Systems (NOS) support DHCP, including Microsoft, Novell, IBM and UNIX platforms. It is relatively easy to implement on any NOS, has been around for some time and is pretty stable. Most Network Operating Systems (NOS) support DHCP, including Microsoft, Novell, IBM and UNIX platforms. It is relatively easy to implement on any NOS, has been around for some time and is pretty stable. There are three methods for DHCP to allocate IP addresses to workstations. There are three methods for DHCP to allocate IP addresses to workstations. Manual allocation Manual allocation Automatic allocation Automatic allocation Dynamic allocation Dynamic allocation In the manual allocation method, the network administrator on the DHCP server manually configures the client's IP address in the server. When the client workstation makes the request for an IP address, the server looks at the MAC address (Media Access Control address; manufacture's unique address of the network card) and assigns the client the manually set IP address. In the manual allocation method, the network administrator on the DHCP server manually configures the client's IP address in the server. When the client workstation makes the request for an IP address, the server looks at the MAC address (Media Access Control address; manufacture's unique address of the network card) and assigns the client the manually set IP address. In the automatic allocation method, the DHCP client workstation is assigned an IP address when it first contacts the DHCP server. In this method the IP address is randomly assigned and is not set in the server. The IP address is permanently assigned to the DHCP client and is not reused by another DHCP client. In the automatic allocation method, the DHCP client workstation is assigned an IP address when it first contacts the DHCP server. In this method the IP address is randomly assigned and is not set in the server. The IP address is permanently assigned to the DHCP client and is not reused by another DHCP client. In the dynamic allocation method, the DHCP server assigns an IP address to a requesting client workstation on a temporary basis. The IP address is leased to the DHCP client for a specified duration of time. When this lease expires, the IP address is revoked from the client and the client is required to surrender the address. If the DHCP client still needs an IP address to perform its functions, it can request another IP address In the dynamic allocation method, the DHCP server assigns an IP address to a requesting client workstation on a temporary basis. The IP address is leased to the DHCP client for a specified duration of time. When this lease expires, the IP address is revoked from the client and the client is required to surrender the address. If the DHCP client still needs an IP address to perform its functions, it can request another IP address

IPv6 32-bit address space is likely to eventually run out 32-bit address space is likely to eventually run out IPv6 extends size to 128 bits IPv6 extends size to 128 bits Main features Main features  classless addresses  multiple levels of aggregation are possible  registry  provider  subscriber  subnet  several flavors of multicast  anycast  interoperability with IPv4

Name resolution Done by name servers Done by name servers  essentially look up a name and return an address Centralized design Centralized design  consistent  single point of failure  concentrates load

DNS Distributed name server Distributed name server A name server is responsible (an authoritative server) for a set of domains A name server is responsible (an authoritative server) for a set of domains May delegate responsibility for part of a domain to a child May delegate responsibility for part of a domain to a child Root servers are replicated Root servers are replicated If local server cannot answer a query, it asks root, which delegates reply If local server cannot answer a query, it asks root, which delegates reply Reply is cached and timed out Reply is cached and timed out

Finding data link layer addresses Data link layer address: most common format is IEEE 802 Data link layer address: most common format is IEEE 802 Need to know data link layer address typically for the last hop Need to know data link layer address typically for the last hop

ARP To get datalink layer address of a machine on the local subnet To get datalink layer address of a machine on the local subnet Broadcast a query with IP address onto local LAN Broadcast a query with IP address onto local LAN Host that owns that address (or proxy) replies with address Host that owns that address (or proxy) replies with address All hosts are required to listen for ARP requests and reply All hosts are required to listen for ARP requests and reply  including laser printers! Reply stored in an ARP cache and timed out Reply stored in an ARP cache and timed out

ARP continued…… The address resolution protocol (arp) is a protocol used by the Internet Protocol (IP), specifically IPv4, to map IP network addresses to the hardware addresses used by a data link protocol. The address resolution protocol (arp) is a protocol used by the Internet Protocol (IP), specifically IPv4, to map IP network addresses to the hardware addresses used by a data link protocol. Internet Protocol (IP)IP network addresses Internet Protocol (IP)IP network addresses The protocol operates below the network layer as a part of the interface between the OSI network and OSI link layer. It is used when IPv4 is used over Ethernet. The protocol operates below the network layer as a part of the interface between the OSI network and OSI link layer. It is used when IPv4 is used over Ethernet. IPv4 is used over Ethernet. IPv4 is used over Ethernet. The term address resolution refers to the process of finding an address of a computer in a network. The address is "resolved" using a protocol in which a piece of information is sent by a client process executing on the local computer to a server process executing on a remote computer. The term address resolution refers to the process of finding an address of a computer in a network. The address is "resolved" using a protocol in which a piece of information is sent by a client process executing on the local computer to a server process executing on a remote computer. The information received by the server allows the server to uniquely identify the network system for which the address was required and therefore to provide the required address. The information received by the server allows the server to uniquely identify the network system for which the address was required and therefore to provide the required address. The address resolution procedure is completed when the client receives a response from the server containing the required address. The address resolution procedure is completed when the client receives a response from the server containing the required address.

. An Ethernet network uses two hardware addresses which identify the source and destination of each frame sent by the Ethernet. An Ethernet network uses two hardware addresses which identify the source and destination of each frame sent by the Ethernet.Ethernet The destination address (all 1's) may also identify a broadcast packet (to be sent to all connected computers). The destination address (all 1's) may also identify a broadcast packet (to be sent to all connected computers).broadcast The hardware address is also known as the Medium Access Control (MAC) address, in reference to the standards which define Ethernet. Each computer network interface card is allocated a globally unique 6 byte link address when the factory manufactures the card (stored in a PROM). This is the normal link source address used by an interface. The hardware address is also known as the Medium Access Control (MAC) address, in reference to the standards which define Ethernet. Each computer network interface card is allocated a globally unique 6 byte link address when the factory manufactures the card (stored in a PROM). This is the normal link source address used by an interface.Medium Access Control (MAC) addressEthernetnetwork interface cardMedium Access Control (MAC) addressEthernetnetwork interface card A computer sends all packets which it creates with its own hardware source link address, and receives all packets which match the same hardware address in the destination field or one (or more) pre-selected broadcast/multicast addresses. A computer sends all packets which it creates with its own hardware source link address, and receives all packets which match the same hardware address in the destination field or one (or more) pre-selected broadcast/multicast addresses. The Ethernet address is a link layer address and is dependent on the interface card which is used. The Ethernet address is a link layer address and is dependent on the interface card which is used. IP operates at the network layer and is not concerned with the link addresses of individual nodes which are to be used. IP operates at the network layer and is not concerned with the link addresses of individual nodes which are to be used.link addresseslink addresses The address resolution protocol (arp) is therefore used to translate between the two types of address. The arp client and server processes operate on all computers using IP over Ethernet. The processes are normally implemented as part of the software driver that drives the network interface card. The address resolution protocol (arp) is therefore used to translate between the two types of address. The arp client and server processes operate on all computers using IP over Ethernet. The processes are normally implemented as part of the software driver that drives the network interface card.IP over Ethernet network interface cardIP over Ethernet network interface card

Figure 20.2 ARP operation

Figure 20.3 ARP packet

Figure 20.4 Encapsulation of ARP packet

Figure 20.5 Four cases using ARP

An ARP request is broadcast; an ARP reply is unicast. Note:

Example 1 A host with IP address and physical address B has a packet to send to another host with IP address and physical address A46EF45983AB. The two hosts are on the same Ethernet network. Show the ARP request and reply packets encapsulated in Ethernet frames. Solution Figure 20.6 shows the ARP request and reply packets. Note that the ARP data field in this case is 28 bytes, and that the individual addresses do not fit in the 4-byte boundary. That is why we do not show the regular 4-byte boundaries for these addresses. Note that we use hexadecimal for every field except the IP addresses.

Figure 20.6 Example 1