InternetworkingENEE 426 | Communication Networks | Spring 2008 Lecture 15Class so far…• Covered individual networks– PHY– MAC / Data Link• Point-to-point networks• Point-to-multipoint networks• Multipoint-to-multipoint networks• Internetworking– Networks of networks– Rest of the semester focuses on L3/L4 issues with networksENEE 426 | Communication Networks | Spring 2008 Lecture 15Connecting networks• Switches connect together components of a single network– Components can have different PHY but must have same MAC• Routers connect together independent networks– Components can have different PHY/MAC but must have same network layer (L3)• Internet Protocol (IP) is common L3 across the entire Internet, and most private networks todayENEE 426 | Communication Networks | Spring 2008 Lecture 15Internetworking Example: Home RouterRouterEthernetSwitchDOCSISWiFiAccess PointWiFi has compatible MAC with EthernetBoth CSMAWiFi AP can bridge the two networksWithout using the Router directlyENEE 426 | Communication Networks | Spring 2008 Lecture 15Internet Protocol (IP)• IP version 4– Global addressing– Best effort traffic– Fragmentation and reassembly• IP version 6– Larger address space (2^128 versus 2^32)– Built-in device auto-configuration– IPsec built-in (L3 security)– Larger MTUs– Multicast built-in– Mobility built-inENEE 426 | Communication Networks | Spring 2008 Lecture 15Global Addressing• IP address: 4 bytes = 32 bits• Typically represented in “dotted-decimal notation” with each byte as a decimal integer, separated by dots– aaa.bbb.ccc.ddd where aaa, bbb, ccc, ddd are [0, 255]– First portion of the address represents the network– Last portion of the address represents the individual hostClassLeadingNet bitsHost bitsNet countHost countNetA072412616,777,2140-127B10141616,38265,534128-191C1102182,097,150254192-223D1110Multicast224-239E1111Reserved240-255ENEE 426 | Communication Networks | Spring 2008 Lecture 15ENEE 426 | Communication Networks | Spring 2008 Lecture 15Classful Addressing• Improvement over original scheme with 1-byte network, 3-byte host– Limits Internet to 256 networks• Allowed for many different sizes and scales of networks to be deployed• Around 1993 again an IP address shortage began• Also Class A networks with 2^24 hosts became unmanageableENEE 426 | Communication Networks | Spring 2008 Lecture 15Solution: CIDR• Classless Inter-Domain Routing• Concept of “subnets”– Break large networks up in to smaller ones– Since address can be arbitrary, you need a mask to tell you what part of the IP address represents the network, and what part represents the host– Result: subnet mask• Useful for routing (get to that next week)– Telescoping address, with bits further down in the address representing further sub-networksIP Supporting ProtocolsENEE 426 | Communication Networks | Spring 2008 Lecture 15Internet Control Messaging Protocol• ICMP• Diagnostic protocol• Major Messages– Echo request/response (ping)– Destination unreachable– Time Exceeded (TTL expiration)ENEE 426 | Communication Networks | Spring 2008 Lecture 15Address Resolution Protocol• ARP• Given an IP address, determine MAC address• Broadcast Ethernet message (destination address ff:ff:ff:ff:ff:ff)• Contains source/destionation IP address and src MAC address• Requests for computer with matching IP address to respond back with their MAC address• Information cached in ARP tablesENEE 426 | Communication Networks | Spring 2008 Lecture 15ARP and Subnet Masks• Devices configured with IP address, subnet mask, and router IP address• A wants to talk to B– A computes (IP-A && Mask) and (IP-B && Mask)– If (IP-A && Mask) == (IP-B && Mask) then B is on the same network as A• Send packet to B’s MAC and IP address (ARP for MAC if unknown)– If not equal, B is on a different network• Send packet to B’s IP and Router’s MAC (ARP for MAC if unknown)• On destination network, last-hop router will ARP for B’s MAC to transmit the packet to BENEE 426 | Communication Networks | Spring 2008 Lecture 15Domain Name Service• DNS• IP addresses hard to remember• Need easy to remember names for hosts on the Internet• Fully Qualified Domain Name (FQDN)• Example: www.umd.edu– edu is a “top level domain” or TLD• Fixed number of TLDs, maintained by ICANN• Generic vs Country-Specific– umd is domain name owned by UMD– www is the name of the serverENEE 426 | Communication Networks | Spring 2008 Lecture 15Reverse DNS• Given an IP address, what is the FQDN?• IP address is a.b.c.d, perform lookup on d.c.b.a.arpa• .arpa is a special TLD for reverse DNS• ISPs maintain RDNS tablesENEE 426 | Communication Networks | Spring 2008 Lecture 15Host Configuration• Devices need to know a lot of information about the network in order to use it– IP address, Router, DNS Server, Subnet Mask• Early Solution: Reverse ARP (RARP)– Send packet with source MAC of host to configure, broadcast destination– Requires RARP server on every network, and does not configure extended propertiesENEE 426 | Communication Networks | Spring 2008 Lecture 15Dynamic Host Configuration Protocol• DHCP• Allows hosts to be autoconfigured when they connect to the network• Allows central administration of network connection information• Static and dynamic IP addressallocations• Addresses from a pool, leased forspecified amount of