CMPE 150- Introduction to Computer Networks 1 CMPE 150 Fall 2005 Lecture 25 Introduction to Computer Networks

CMPE 150- Introduction to Computer Networks 2 Announcements Homework 4 due today by midnight. No class on Friday, IMPORTANT: No lab tonight. –Make up lab session next week. Final exam: December 7 th. 4-7pm –In class. –Closed books/notes. Course evaluation. –Need volunteers! CE 151 will be offered in Winter 06!

CMPE 150- Introduction to Computer Networks 3 Today IP (Cont’d).

CMPE 150- Introduction to Computer Networks 4 IP (Internet Protocol) Glues Internet together. Common network-layer protocol spoken by all Internet participating networks. Best effort datagram service: –No reliability guarantees. –No ordering guarantees.

CMPE 150- Introduction to Computer Networks 5 IP Versions IPv4: IP version 4. –Current, predominant version. –32-bit long addresses. IPv6: IP version 6 (aka, IPng). –Evolution of IPv4. –Longer addresses (16-byte long).

CMPE 150- Introduction to Computer Networks 6 IP Datagram Format IP datagram consists of header and data (or payload). Header: –20-byte fixed (mandatory) part. –Variable length optional part.

CMPE 150- Introduction to Computer Networks 7 The IP v4 Header

CMPE 150- Introduction to Computer Networks 8 IP Options 5-54

CMPE 150- Introduction to Computer Networks 9 IP Addresses IP address formats.

CMPE 150- Introduction to Computer Networks 10 IP Addresses (Cont’d) Class A: 128 networks with 16M hosts each. Class B: 16,384 networks with 64K hosts each. Class C: 2M networks with 256 hosts each. More than 500K networks connected to the Internet. Network numbers centrally administered by ICANN.

CMPE 150- Introduction to Computer Networks 11 IP Addresses (Cont’d) Special IP addresses.

CMPE 150- Introduction to Computer Networks 12 Scalability of IP Addresses Problem: a single A, B, or C address refers to a single network. As organizations grow, what happens?

CMPE 150- Introduction to Computer Networks 13 Example: A Campus Network

CMPE 150- Introduction to Computer Networks 14 Solution Subnetting: divide the organization’s address space into multiple “subnets”. How? Use part of the host number bits as the “subnet number”. Example: Consider a university with 35 departments. –With a class B IP address, use 6-bit subnet number and 10-bit host number. –This allows for up to 64 subnets each with 1024 hosts.

CMPE 150- Introduction to Computer Networks 15 Subnets A class B network subnetted into 64 subnets.

CMPE 150- Introduction to Computer Networks 16 Subnet Mask Indicates the split between network and subnet number + host number. Subnet Mask: or /22 (network + subnet part)

CMPE 150- Introduction to Computer Networks 17 Subnetting: Observations Subnets are not visible to the outside world. Thus, subnetting (and how) is a decision made by local network admin.

CMPE 150- Introduction to Computer Networks 18 Subnet: Example Subnet 1: | – Subnet 2: | – Subnet 3: | –

CMPE 150- Introduction to Computer Networks 19 Problem with IPv4 IPv4 is running out of addresses. Problem: class-based addressing scheme. –Example: Class B addresses allow 64K hosts. More than half of Class B networks have fewer than 50 hosts!

CMPE 150- Introduction to Computer Networks 20 Solution: CIDR CIDR: Classless Inter-Domain Routing. –RFC Allocate remaining addresses in variable- sized blocks without considering classes. Example: if an organization needs 2000 addresses, it gets 2048-address block. Forwarding had to be modified. –Routing tables need an extra entry, a 32-bit mask, which is ANDed with the destination IP address. –If there is a match, the packet is forwarded on that interface.

CMPE 150- Introduction to Computer Networks 21 Network Address Translation Another “quick fix” to the address shortage in IP v4. Specified in RFC Each organization gets a single (or small number of) IP addresses. –This is used for Internet traffic only. –For internal traffic, each host gets its own “internal” IP address. Three IP ranges have been declared as “private”. – – /8 – – /12 – – /16 No “private” IP address can show up on the Internet, i.e., outside the organization’s network.

CMPE 150- Introduction to Computer Networks 22 NAT – Network Address Translation

CMPE 150- Introduction to Computer Networks 23 Internet Control Protocols “Companion” protocols to IP. Control protocols used mainly for signaling and exchange of control information. Examples: ICMP, ARP, RARP, BOOTP, and DHCP.

CMPE 150- Introduction to Computer Networks 24 ICMP Internet Control Message Protocol. A way to “debug” the Internet and find out what is happening at routers. Defines a dozen different messages that are generated typically by routers upon some unexpected event.

CMPE 150- Introduction to Computer Networks 25 ICMP Message Types 5-61

CMPE 150- Introduction to Computer Networks 26 Address Resolution Protocol ARP. RFC 826. Protocol for machines to map IP addresses to Ethernet addresses. –This is needed when packet needs to be delivered to a local host on a LAN (Ethernet).

CMPE 150- Introduction to Computer Networks 27 ARP: Example. Host 1 wants to send packet to host 2.. Assume that host 1 knows host 2’s IP address.. Host 1 builds packet with host 2’s IP address.. IP knows it’s a local destination but now needs host 2’s Ethernet address.

CMPE 150- Introduction to Computer Networks 28 ARP Operation Host 1 broadcasts an ARP request on the Ethernet asking who owns host 2’s IP address. Host 2 replies with its Ethernet address. Some optimizations: –ARP caches. –Piggybacking host’s own Ethernet address on ARP requests. –Proxy ARP: services ARP requests for hosts on separate LANs.

CMPE 150- Introduction to Computer Networks 29 Beyond ARP ARP solves the problem of mapping IP address to Ethernet address. How do we solve the inverse problem? –I.e., how to map an Ethernet address to an IP address? Older protocols: RARP (RFC 903) and BOOTP (RFC 951). –RARP broadcasts not forwarded by routers. –BOOTP uses UDP but requires manual configuration of IP-Ethernet mappings.

CMPE 150- Introduction to Computer Networks 30 DHCP Dynamic Host Configuration Protocol. RFCs 2131 and Assigns IP addresses to hosts dynamically. DHCP server may not be on the same LAN as requesting host. DHCP relay agent.

CMPE 150- Introduction to Computer Networks 31 DHCP Operation Newly booted host broadcasts a DHCP DISCOVER message. DHCP relay agent intercepts DHCP DISCOVERs on its LAN and unicasts them to DHCP server.

CMPE 150- Introduction to Computer Networks 32 DHCP Operation

CMPE 150- Introduction to Computer Networks 33 DHCP: Address Reuse How long should an IP address be allocated? Issue: hosts come and go. IP addresses may be assigned on a “Lease” basis. Hosts must renew their leases.