4: Network Layer4a-1 IP Addressing: introduction r IP address: 32-bit identifier for host, router interface r interface: connection between host, router and physical link m routers typically have multiple interfaces m host may have multiple interfaces m IP addresses associated with interface, not host, router =

4: Network Layer4a-2 IP Addressing r IP address: m network part (high order bits) m host part (low order bits) r What’s a network ? ( from IP address perspective) m device interfaces with same network part of IP address m can physically reach each other without intervening router network consisting of 3 IP networks (for IP addresses starting with 223, first 24 bits are network address) LAN

4: Network Layer4a-3 IP Addressing How to find the networks? r Detach each interface from router, host r create “islands of isolated networks” Interconnected system consisting of six networks

4: Network Layer4a-4 IP Addresses 0 network host 10 network host 110 networkhost 1110 multicast address A B C D class to to to to bits given notion of “network”, let’s re-examine IP addresses: “class-full” addressing:

4: Network Layer4a-5 IP addressing: CIDR r classfull addressing: m inefficient use of address space, address space exhaustion m e.g., class B net allocated enough addresses for 65K hosts, even if only 2K hosts in that network r CIDR: Classless InterDomain Routing m network portion of address of arbitrary length m address format: a.b.c.d/x, where x is # bits in network portion of address network part host part /23

4: Network Layer4a-6 IP addresses: how to get one? Hosts (host portion): r hard-coded by system admin in a file r DHCP: Dynamic Host Configuration Protocol: dynamically get address: “plug-and-play” m host broadcasts “DHCP discover” msg m DHCP server responds with “DHCP offer” msg m host requests IP address: “DHCP request” msg m DHCP server sends address: “DHCP ack” msg

4: Network Layer4a-7 IP addresses: how to get one? Network (network portion): r get allocated portion of ISP’s address space: ISP's block /20 Organization /23 Organization /23 Organization /23... ….. …. …. Organization /23

4: Network Layer4a-8 Hierarchical addressing: route aggregation “Send me anything with addresses beginning /20” / / /23 Fly-By-Night-ISP Organization 0 Organization 7 Internet Organization 1 ISPs-R-Us “Send me anything with addresses beginning /16” /23 Organization Hierarchical addressing allows efficient advertisement of routing information:

4: Network Layer4a-9 Hierarchical addressing: more specific routes ISPs-R-Us has a more specific route to Organization 1 “Send me anything with addresses beginning /20” / / /23 Fly-By-Night-ISP Organization 0 Organization 7 Internet Organization 1 ISPs-R-Us “Send me anything with addresses beginning /16 or /23” /23 Organization

4: Network Layer4a-10 IP addressing: the last word... Q: How does an ISP get block of addresses? A: ICANN: Internet Corporation for Assigned Names and Numbers m allocates addresses m manages DNS m assigns domain names, resolves disputes

4: Network Layer4a-11 Getting a datagram from source to dest. IP datagram: A B E misc fields source IP addr dest IP addr data r datagram remains unchanged, as it travels source to destination r addr fields of interest here Dest. Net. next router Nhops routing table in A

4: Network Layer4a-12 Getting a datagram from source to dest A B E Starting at A, given IP datagram addressed to B: r look up net. address of B r find B is on same net. as A r link layer will send datagram directly to B inside link-layer frame m B and A are directly connected Dest. Net. next router Nhops misc fields data

4: Network Layer4a-13 Getting a datagram from source to dest A B E Dest. Net. next router Nhops Starting at A, dest. E: r look up network address of E r E on different network m A, E not directly attached r routing table: next hop router to E is r link layer sends datagram to router inside link- layer frame r datagram arrives at r continued….. misc fields data

4: Network Layer4a-14 Getting a datagram from source to dest A B E Arriving at , destined for r look up network address of E r E on same network as router’s interface m router, E directly attached r link layer sends datagram to inside link-layer frame via interface r datagram arrives at !!! (hooray!) misc fields data network router Nhops interface Dest. next

4: Network Layer4a-15 IP datagram format ver length 32 bits data (variable length, typically a TCP or UDP segment) 16-bit identifier Internet checksum time to live 32 bit source IP address IP protocol version number header length (32 bit words) max number remaining hops (decremented at each router) for fragmentation/ reassembly total datagram length (bytes) upper layer protocol to deliver payload to head. len type of service “type” of data flgs fragment offset upper layer 32 bit destination IP address Options (if any) E.g. timestamp, record route taken, pecify list of routers to visit.

4: Network Layer4a-16 IP Fragmentation & Reassembly r network links have MTU (max.transfer size) - largest possible link-level frame. m different link types, different MTUs r large IP datagram divided (“fragmented”) within net m one datagram becomes several datagrams m “reassembled” only at final destination m IP header bits used to identify, order related fragments fragmentation: in: one large datagram out: 3 smaller datagrams reassembly

4: Network Layer4a-17 IP Fragmentation and Reassembly ID =x offset =0 fragflag =0 length =4000 ID =x offset =0 fragflag =1 length =1500 ID =x offset =1480 fragflag =1 length =1500 ID =x offset =2960 fragflag =0 length =1040 One large datagram becomes several smaller datagrams