IP Addressing

Internet Protocol: Which version? There are currently two versions of the Internet Protocol in use for the Internet IPv4 Specified in 1980/81 (RFC 760, 791) Four byte addresses Universally deployed Problem: Address space almost exhausted IPv6 Specification from 1998 (RFC 2460) Significant differences to IPv4, but not fundamental changes 16-byte addresses Problem: Not widely used (yet?) Google Study from Oct 2008: IPv6 available to 0.238% of users

IP Addresses

What is an IP Address? IP address = Internet Protocol Version 4 address 32-bit label for a network interface –Each interface has a separate IP address On the public Internet, IP address is unique global address The IP address is to used by hosts and routers for delivery of IP datagram Important special cases: –Dynamically assigned IP addresses ( DHCP) –IP addresses in private networks ( NAT)

Dotted Decimal Notation IPv4 addresses are written using dotted decimal notation Each byte is identified by a decimal number in the range [0..255] Example: st Byte = nd Byte = rd Byte = th Byte =

IP address consists of a network prefix and a host number –Network prefix identifies a network –Host number identifies an interface on the network How do we know how long the network prefix is? –The length of the network prefix is indicated 1.by a netmask, or 2.by a suffix given the length as a number (CIDR notation) –Netmask is often called subnetmask (there are subtle differences) Structure of an IP address network prefixhost number

Example: Network address is: (or ) Host number is: Netmask is: (or ffff0000) Prefix or CIDR notation: /16 »Network prefix is 16 bits long Notation of IP address

Network Prefix and Host Number Each IP network (often called subnetwork or subnet) has an IP address: IP address of a network = Host number is set to all zeros, e.g., IP routers are devices that forward IP datagrams between IP networks Delivery of an IP datagram proceeds in 2 steps: –Use network prefix to deliver datagram to the right network –Once the network is found, use the host number to deliver to the right interface

How does one get an IP network address? IP address allocation is managed by five Regional Internet Registries (RIR) Each RIR manages ranges of addresses:

Special IP Addresses Special addresses: Loopback interfaces –all addresses are reserved for loopback interfaces –Most systems use as loopback address –loopback interface is associated with name localhost IP address of a network –Host number is set to all zeros, e.g., Broadcast address –Host number is all ones, e.g., –Broadcast goes to all hosts on the network –Often ignored due to security concerns Test / Experimental addresses Certain address ranges are reserved for experimental use. Packets should get dropped if they contain this destination address (see RFC 1918): Convention (but not a reserved address) Default gateway has host number set to 1, e.g., e.g.,

Subnetting Scenario: Organization has a large network prefix and wants to delegate management of IP addresses Subnetting: Use a portion of the host name to identify a smaller network ( subnetwork, subnet). Each subnets is a separate IP network UofT Network Faculty of A&S Library Faculty of Engineering / / / /24

Basic Idea of Subnetting Split the host number portion of an IP address into a subnet number and a (smaller) host number. Result is a 3-layer hierarchy The extended network prefix is also called subnetmask Then: Subnets can be freely assigned within the organization Internally, subnets are treated as separate networks Subnet structure is not visible outside the organization network prefixhost number subnet number network prefix host number extended network prefix

Routers and hosts use an extended network prefix (subnetmask) to identify the start of the host numbers Subnetmask

Example: Subnetmask /16 is the IP address of the network /24 is the IP address of the subnet is the IP address of the host (or ffffff00) is the subnetmask of the host When subnetting is used, one generally speaks of a subnetmask (instead of a netmask) and a subnet (instead of a network)

Advantages of Subnetting With subnetting, IP addresses use a 3-layer hierarchy: »Network »Subnet »Host Reduces router complexity. Since external routers do not need to know about subnetting, the complexity of routing tables at external routers is reduced. Note: Length of the subnet mask need not be identical at all subnetworks.

Creating and Managing Subnetting Use of subnetting or length of the subnetmask is decided by the network administrator, who acts as owner of Subnets are created by setting the subnetmask of an IP interface Important: Subnetmasks must be set consistently

No Subnetting All hosts think that the other hosts are on the same network

With Subnetting Hosts with same extended network prefix belong to the same network

Different subnetmasks lead to different views of the scope of the network With Subnetting

Classful IP Addresses and CIDR In 1993, there was a major shift for interpreting and allocating IP addresses: until 1993: Classful Addresses until 1993: Classful Addresses after 1993: CIDR = Classless Interdomain Routing after 1993: CIDR = Classless Interdomain Routing

Classful IP Addresses When Internet addresses were standardized (early 1980s), the Internet address space was divided up into classes: –Class A: Network prefix is 8 bits long –Class B: Network prefix is 16 bits long –Class C: Network prefix is 24 bits long –Class D: Used (multicast) group addresses –Class E: Reserved Each IP address contained a key which identifies the class: –Class A: IP address starts with 0 –Class B: IP address starts with 10 –Class C: IP address starts with 110 No need for netmasks (unless subnetting is used)

Classful IP Addresses

We will learn about multicast addresses later in this course.

IP Allocation with Classful IP Addresses Limited flexibility for obtaining a network address: 254 IP addresses: Class C = IP addresses:Class B = 16,772,214 IP addresses:Class A Too few network addresses for large networks 2 7 = 128 Class A networks 2 14 = 16,384 Class B networks Flat address space. Routing tables in the backbone Internet need one entry for each network address. Class C networks: up to 2 21 = 2,097,152 entries By 1993, the lookup time of routing tables had become a bottleneck for Internet performance

Allocation of Classful Addresses

CIDR - Classless Interdomain Routing Goals: –New interpretation of the IP address space –Restructure IP address assignments to increase efficiency –Enable routing table aggregation to minimize route table entries CIDR (Classless Interdomain Routing) –Abandons the notion of classes –Key Concept: The length of the network prefix in the IP addresses is kept arbitrary –Consequences: Size of the network prefix must be provided with an IP address ( CIDR notation of IP addresses) Hierarchical routing aggregation introduces dependency of IP addresses to service provider Need for new lookup algorithms for routing tables

CIDR Notation CIDR notation of an IP address: /18 "18" is the prefix length. It states that the first 18 bits are the network prefix of the address (and 14 bits are available for specific host addresses) CIDR notation can replace the use of subnetmasks (but is more general) –IP address and subnetmask becomes /24 CIDR notation allows to drop traling zeros of network addresses: /18 can be written as /18

CIDR address blocks CIDR notation can nicely express blocks of addresses Blocks are used when allocating IP addresses for a company and for routing tables (route aggregation) CIDR Block Prefix # of Host Addresses /2732 /2664 /25128 /24256 /23512 /221,024 /212,048 /204,096 /198,192 /1816,384 /1732,768 /1665,536 /15131,072 /14262,144 /13524,288

Subnetting and Supernetting CIDR is compatible with subnetting: –Subnets are created by extending the prefix CIDR can do more: –CIDR can refer to multiple networks with a single prefix: /16 and /16 can be summarized as /15 –This is called supernetting (In fact, CIDR and supernetting are often used as the same thing) –If neighboring networks have similar address blocks, supernetting reduces the size of routing tables

CIDR and Hierarchical IP address allocation Exploiting supernetting to reduce size of routing tables: –Backbone ISPs obtain blocks of IP addresses and allocate portions of their address blocks to their customers –Customers can allocate a portion of their address block to their own customers

Hierarchical Structure of IP Addresses /16 Rest of Internet ISP X owns: Company Y : ISP Z : Organization A: Organization B:

Hierarchical Structure of IP Addresses /16 Rest of Internet ISP X owns: Company Y : /24 ISP Z : /24 Organization A: /26 Organization B: /26 Obtained from ARIN Obtained from ISP X Obtained from ISP Z

Hierarchical Structure of IP Addresses /16 Rest of Internet ISP X owns: Company Y : /24 ISP Z : /24 Organization A: /26 Organization B: /26 Only one routing entry for /16 No entries needed for network addresses of Company Y, ISP Z, or Organizations A, B. Routing table entry for /24 and /24 (no entry for /26 and /26.

Drawback of Hierarchical IP Addresses IP address assignment depends on service provider /16 Rest of Internet ISP X owns: Company Y : /24 ISP Z : /24 Organization A: /26 Organization B: /26 Must change IP addresses when changing service provider /26

CIDR and Routing Tables Aggregation of routing table entries: – /16 and /16 can be represented as /15 Longest prefix match: Routing table lookup finds the routing entry that matches the the longest prefix What is the outgoing interface for /24 ? Route aggregation can be exploited when IP address blocks are assigned in an hierarchical fashion PrefixInterface /4interface # /9interface # /17interface #1 Routing table

IPv6 - IP Version 6 IP Version 6 –Is the successor to the currently used IPv4 –Specification completed in 1994 –Makes improvements to IPv4 (no revolutionary changes) One (not the only !) feature of IPv6 is a significant increase in of the IP address to 128 bits (16 bytes) IPv6 will solve – for the foreseeable future – the problems with IP addressing addresses per square inch on the surface of the Earth.

IPv6 Header

IPv6 vs. IPv4: Address Comparison IPv4 has a maximum of billion addresses IPv6 has a maximum of = (2 32 ) 4 4 billion x 4 billion x 4 billion x 4 billion addresses

Notation of IPv6 addresses Convention: The 128-bit IPv6 address is written as eight 16- bit integers (using hexadecimal digits for each integer) CEDF:BP76:3245:4464:FACE:2E50:3025:DF12 Short notation: Abbreviations of leading zeroes: CEDF:BP76:0000:0000:009E:0000:3025:DF12 CEDF:BP76:0:0:9E :0:3025:DF12 :0000:0000:0000 can be written as :: CEDF:BP76:0:0:FACE:0:3025:DF12 CEDF:BP76::FACE:0:3025:DF12 IPv6 addresses derived from IPv4 addresses have 96 leading zero bits. Convention allows to use IPv4 notation for the last 32 bits. ::80:8F:89:90 ::