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CMPT 371

CMPT 371. Data Communications and Networking Network Layer Addressing and forwarding ( classful , CIDR, IPv4 ) . Identifying Hosts. An IP address is associated with a network interface (for example ethernet card) attached to a host or router.

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CMPT 371

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  1. CMPT 371 Data Communications and Networking Network Layer Addressing and forwarding (classful, CIDR, IPv4)

  2. Identifying Hosts • An IP address is associated with a network interface (for example ethernet card) attached to a host or router. • A host/router with more than one network interface will have more than one IP address. In fact a router needs more than one network interface so it can receive a packet on one interface and send it on another interface • A host is identified by one IP address. It has one interface to a single network • A multi-homed host (may or may not be a router) or a router has multiple IP addresses and usually connects to multiple networks. • Each network interface connected to the host/router has a unique IP address

  3. Internet addresses: IPv4 • 32 bit global internet (IP) address is used to uniquely identify a particular network interface connected to a particular host as a destination for communication • Globally applicable and globally unique • Expressed a series of 32 binary digits • 10000000 00001011 00000011 00011111 • Also expressed in dotted decimal notation • Binary digits are separated into four groups of eight digits • Each group of 8 digits are translated to a decimal number • The decimal number are separated by dots (periods) • Example address above becomes 128.11.3.31

  4. Structure of an IP address • Each IP address is split into two parts (netid, hostid) to identify the host and the network to which the host is connected • The netid (network address or prefix) identifies the network to which the host belongs. • The number of bits dedicated to the netid will determine the number of possible networks. • The hostid identifies the particular host (network interface for a multi homed host) • The number of bits dedicated to the hostid will determine the possible number of hosts on the network

  5. Prefix notation: IP addresses • To indicate the length of the prefix associated with a particular IP address use the notation • 178.23.214.0/22 ⇨ prefix with n=22 binary digits • 178.23.214.0/24 ⇨ prefix with n=24 binary digits • The prefix consists of the first n binary digits of the address • The prefix often indicates the netid of a network. If it does then 232-n indicates the number of possible hosts in the network (or subnet)

  6. Network address or Network prefix • Netid (network address) non zero: hostid all 0’s • never assigned as the source or destination address of an IP packet, or as the address or a single host/router • Used in forwarding tables and documentation to refer to all hosts on a particular network • A network address is assigned to the network itself, not to an individual host or router • The network address defines the network to the rest of the internet • If an IP address has a netid corresponding to the address of a particular network then that the IP address is the address of a host on that particular network

  7. Sample Networks 223.12.1.1 223.12.8.1 223.12.0.0/22 223.12.8.0/24 223.12.2.5 223.12.8.33 223.12.10.0/23 223.12.3.254 223.12.8.88 223.12.11.251 223.12.10.21 223.12.11.2

  8. “this” address • Netid (network address) zero: hostid nonzero • Interpreted as hostid on “this” network • 0.0.0.0 • “this” host when network address is also unknown • Used only when booting a host that does not know its own IP address (usually a diskless host)

  9. Broadcast address • Network broadcast addresses are valid only as a destination • directed broadcast:broadcast to all stations on the local network from anywhere reached by the internet • netid is network address for the network • hostid all 1’s • A security risk for denial of service attacks, by default directed broadcast is disabled • limited broadcast or local network broadcast: broadcast to all stations on the local network from within that local network • netid and hostid all 1’s. • May be used when node starts to establish its IP address

  10. Loopback address • Address used to send packets from one process to another through the local interface within a host • Packets sent to the loopback address will not leave the local host, they will never be sent onto any network • Packets sent to the loopback address will pass through the local interface (lo) • Available loopback addresses 127.0.0.0 to 127.255.255.254, usually use 127.0.0.1

  11. Private or Non-Routable addresses • Some addresses are reserved for use on local networks that are not connected to the Internet • Routers do not consider these addresses to be valid Internet addresses, and will not route a packet to any of them • These addresses may be used on private internets not directly connected to the Internet. • 10.0.0.0/8 10.0.0.0 to 10.255.255.255 • 172.16.0.0/12 172.16.0.0 to 172.31.255.255 • 192.168.0.0/16 192.168.0.0 to 192.168.255.255

  12. Allocating addresses to networks • Have considered some addresses reserved for particular purposes. • How are the remainder of the addresses in the IP address space allocated to networks? • Originally, the IPv4 protocol originally separated addresses into different classes, allowing for particular numbers of networks in each class. The addressing was know as classful addressing • Later, when the number of networks began to exceed the available network addresses an extended solution was needed. The solutions implemented were • Long term solution: new version of the IP protocol IPv6 • Short term solution classless addressing or CIDR

  13. Classful addressing

  14. Classful Addressing: forwarding • The original forwarding algorithms depended on each network having a network address that was either a Class A, B, C, D, or E address. • Each network would have one entry in the forwarding table of each router. • The entry would indicate the network address of the destination network and the interface on the present router through which the packet should be sent to reach that destination network. • The incoming packets destination address would be compared to all entries (of the correct class) in the forwarding table to determine the correct forwarding table entry and hence the interface through which the packet should be forwarded

  15. Why Subnets? • Large networks were difficult to administer and needed some internal structure to simplify their administration. • Allow arbitrary complexity of internetworked LANs within organization (with same external netid) • Many LANs all with the same external netid • Each LAN with its own local subnetid • Insulate overall internet from growth of network numbers and routing complexity • Site looks like a single network to rest of the internet

  16. How to use Subnet Masks • A site (with 1 or more routers connecting it to the internet) using a single netid has several local LANs. • The site administrator must decide how many LANs are/ may be needed within the installation (the single netid). • If M LANs are needed then choose N such that M<2N-2 • Each LAN assigned subnet id between 1 and M, this is added to the network address to give the subnet address • Host portion of address partitioned into subnet number and host number, The N higher order bits are the subnet number. • Local routers route within subnetted network • Subnet mask indicates which bits are subnet number and which are host number

  17. Subnetting: Example 133.12.64.0/19 133.12.66.1 133.12.75.52 133.12.94.25 133.12.128.0/19 133.12.160.0/19 133.12.159.252 133.12.128.21 133.12.138.23 133.12.0.0/16 Internet 133.12.162.29 133.12.191.254 133.12.168.33

  18. Subnetting example (1) • The site illustrated has one router connecting it to the internet. • The netid of that router as seen from the internet is a class B network address, 133.12.0.0/24 • The local network behind the router consists of several different internal networks • The site administrator for these networks must decide how many subnets are / may be needed within the installation (the single netid). • For this example up to 6 networks are needed • 6 LANs, choose N such that 6<2N , N=3, M=2N=8 • First 3 of the 16 bits available for hostid will be used to indicate which subnet the host belongs to leaving 13 bits for the hostid

  19. Subnetting example (2) • Each LAN assigned subnet id between 1 and M=8, this is added to the network address to give the subnet address • The three subnets illustrated are • Subnet 2, (64, binary 01000000), 133.12.64.0-133.12.95.255 • Subnet 4 (128, binary 10000000), 133.12.128.0-133.12.159.255 • Subnet 5 (160, binary 10100000), 133.12.160.0-133.12.191.255 • Host portion of address partitioned into subnet number and host number, The 3 highest order bits are the subnet number, the remaining 13 bits are for the host id (5 bits in the octet shown above plus the 8 bits in the final octet)

  20. Subnetting example (3) • Subnet mask indicates which bits are subnet number and which are host number, for this example the subnet mask will be • 11111111 11111111 11100000 00000000 • The local router will use the subnet mask to determine which subnet an incoming packet is destined for net mask 255.255.0.0 Subnet mask 255.255.224.0 Host id

  21. Using network masks • Consider that the packet to be forwarded has IP address 133.12.138.23 • 10000101 00001100 10001010 00010111 • The netmask of the network is 255.255.224.0 • 11111111 11111111 00000000 00000000 • AND IP address and netmask to give Netid 133.12.0.0

  22. Using subnet masks • Network 133.12.0.0 is broken into smaller subnets by the adminstrator for that network • The adminstrator for 133.12.0.0 defines a Subnet mask 255.255.224.0 • 11111111 11111111 11100000 00000000 • Masks 3 additional bits to create 23=8 subnets • AND with IP address to give subnetwork address 133.12.128.0 • The final 13 bits are reserved for hostid on each subnet • Out example IP has hostid 01010 00010111

  23. Subnetting The subnet address of the zero subnet (subnet id all zero) is the same as the network address for the entire network. The broadcast address of the all 1’s subnet (subnet id all zero) is the same as the broadcast address of the entire network For many years these networks were not used to avoid these ambiguities. They can be used in most cases

  24. Problems with classful • Large networks were difficult to administer and needed some internal structure to simplify their administration. (solution subnetting) • With the explosive growth of the Internet Class B networks were in short supply. • Many organizations wanted more addresses than a class C address could supply but not as many as a class B address would give. • Giving multiple class C addresses was one solution but it had its own problems, increasing the load on the network due to routing (one table entry for each class C network) • Short term solution CIDR, NAT • long term solution IPv6

  25. Classless InterDomain Routing • CIDR (also called supernetting) Permits allocation of the remaining IP addresses in blocks more closely matched to user needs (any prefix not just 8, 16, 24) • Makes forwarding algorithms more complex (cannot sort by class to simplify forwarding, to many prefixes) • Addresses are allocated based on a base address and a prefix, for example 202.25.8.0/22 • 202.25.8.0 is the first allocated address or the network address • The prefix indicates the netmask. A prefix of 22 indicates 22 1’s followed by 10 (32-22) 11111111 11111111 11111100 00000000,

  26. CIDR: Example for EngCO • EngCO has been allocated a block of addresses 196.74.0.0/17 (2(32-17)=32768 addresses) • 196.74.4.0 to 196.74.127.255 • The subnets EngCO has already allocated are 196.74.32.0 to 196.74.35.255 (196.74.32.0/22) 232-22 =1024 addresses, netmask255.255.252.0 (22 1 bits) 196.74.16.0 to 196.74.23.255 (196.74.16.0/21) 232-21=2048 addresses, netmask255.255.248.0 (21 1 bits) 196.74.48.0 to 196.74.63.255 (196.74.48.0/20) 232-20=4096 addresses, netmask255.255.240.0 (20 1 bits)

  27. Addresses for hosts • Consider the network 196.74.48.0/20 • 232-20=4096 addresses • Netmask255.255.240.0 (20 1 bits) • Addresses 196.74.48.0 to 196.74.63.255 • Network address 196.74.48.0 cannot be used for a host because it is the network address • Network broadcast address is 196.74.63.255, so this address cannot be used for a host • So only 232-20 - 2=4096-2=4094 addresses can be used for hosts

  28. Allocated / available space The network address must fall on a 2N boundary where 32-N is the prefix of the network. • 0 indicates address 196.74.0.0 • 12 indicates address 196.74.12.0 196.74.32.0/22 1024 1024 0 48 52 4 40 28 60 12 20 64 16 56 8 44 24 36 32 196.74.16.0/21 2048 2048 • 0 0 48 52 4 40 28 60 12 20 64 16 56 8 44 24 36 32 196.74. 48.0/20 4096 4096 0 48 52 4 40 28 60 12 20 64 16 56 8 44 24 36 32

  29. EngCo’snetworks • To Internet To Internet 196.74.0.0/18 eth0 Router0 eth1 eth3 eth2 196.74.16.0/21 196.74.32.0/22 196.74.48.0/20

  30. A sample forwarding table: Router 0

  31. Hierarchical addressing: 1 • CIDR is a hierarchical addressing approach • Groups of networks can be aggregated to appear as a single network to more distant routers • Entries that appear to be a single network to a particular router may in fact be aggregations of many smaller networks

  32. Hierarchical addressing: 2 • CIDR (RFC 1518, 1519) • Points out that CIDR replaces both sub and super netting, so long as addresses are assigned in blocks with size equal to an integer power of 2 network and host portions are readily separated with a mask • IANA (the organization in charge of administering distribution of IP addresses) has three regional registries • ARIN: North America • RIPE: Europe • APNIC: Asia • LACNIC: South America • Each of these registries was given a large block of addresses

  33. Hierarchical addressing: 3 • Each of the regional registries grants blocks of addresses to each country in its region • Each country may grant addresses on a regional basis within the country • Each country or region of a country will grant addresses to large IP providers and or companies for their networks • These providers or companies apportion addresses to their users

  34. A sample forwarding table: Router 0 • Routers outside EngCowill see the networks EngCo’s networks as a single network. • To reach EngCothey may have a single entry for Destination 196.74.0.0 with netmask 255.255.192.0 and gateway set to the address of the router that sits between ABCEngCo’s networks and the internet.(router 0) • More distant routers may aggregate this entry with others to form a single entry

  35. EngCo’snetworks • To Internet To Internet 196.74.0.0/18 eth0 Router1 Router0 eth1 eth3 eth2 196.74.16.0/21 196.74.32.0/22 196.74.48.0/20

  36. Aggregation of networks • 64*256=16384=232-14 196.74.0.0/18 196.74.32.0/22 196.74.16.0/21 196.74. 48.0/20 1024 1024 0 48 52 4 40 28 60 12 20 64 16 56 8 44 24 36 32 196.74.0.0/18 1024 1024 0 48 52 4 40 28 60 12 20 64 16 56 8 44 24 36 32

  37. CIDR: Routing and aggregation • In order to reduce the size of forwarding tables aggregation is used. Networks in a given region/location are aggregated into a larger network for the purpose of forwarding. • The three networks at EngCo might be aggregated into one router table entry 196.74.0.0/18 in routers (like router 1) outside of EngCo • In more distant routers the above entry might be aggregated into a still larger single entry, for example 196.0.0.0/8

  38. The IPv4 forwarding algorithm • Extract the IP destination address from the packet • For each forwarding table entry use the mask (bitwise AND the mask with the destination IP address) to extract the prefix from the destination address and compare it to the prefix in the table. Remember any entries that match • Choose the matching entry with the longest prefix match • If there is no match send a routing error back to the source

  39. Forwarding example • Consider that router A has the forwarding table on the next slide. • A packet with IP destination address198.53.2.7 arrives at the router A • For each entry (row) in the forwarding table • bitwise AND the destination address with the netmask • Compare the result to the network address in that row • If they match remember that the row matched

  40. A sample IPv4 forwarding table ROUTER A

  41. Using a netmask to extract netid • Destination IP address198.55.2.7 converted to binary • 11000110 00110101 00000010 00000111 • Netmaskof first row 255.255.255.0 converted to binary • 11111111 11111111 11111111 00000000 • AND IP address and Netmask 11000110 00110111 00000010 00000111 11111111 11111111 11111111 00000000 11000110 00110111 00000010 00000000 • Convert result of and to dotted decimal to get the network address 198.55.2.0 does not match network address in the forwarding table entry

  42. Using a netmask to extract netid • Repeat for each successive row, no match until row 4 • Netmaskof 4th row 255.255.192.0 converted to binary • 11111111 11111111 11000000 00000000 • AND IP address and Netmask 11000110 00110101 00000010 00000111 11111111 11111111 11000000 00000000 11000110 00110101 00000000 00000000 • Convert result of and to dotted decimal to get the network address 198.55.0.0 matches the network address in the forwarding table entry • No more matches after row 4

  43. Forwarding: using chosen entry • Once a particular entry (row) in the forwarding table has been selected • Extract the gateway address for the entry, 198.55.1.2, this is the address of the next host/router along the path to the destination. This Ethernet address of this host/router will be the next hop destination of the Ethernet packet containing this IP datagram. • Extract the interface, ETH0, this tells the IP stack which interface (Ethernet card) to send the IP datagram through to reach the next hop gateway or destination

  44. A second example: same table The next packet has an IP destination address 196.16.30.138 AND this IP destination address with the mask in row 2 and you will get the network address in row 2 AND this IP destination address with the mask in row 3 and you will get the network address in row 3 WHAT HAPPENS WHEN 2 ROWS MATCH?

  45. Second Example: longest match • WHAT HAPPENS WHEN 2 ROWS MATCH? • Consider each of the matching entries. • Determine how many bits of the destination IP match the network address of each matching forwarding table entry. • For row 2 mask is 255.255.252.0, or 22 matching digits • For row 3 mask is 255.255.240.0, or 20 matching digits • Choose the entry with the “longest” match, that is the longest mask. Choose row 2. • To optimize the process, entries in the forwarding table are placed in order, starting with the longest masks and continuing with successively shorter matches. • Ordering the entries means the first matching entry is the “longest” match

  46. Historic network: aggregation • Some blocks of addresses were allocated using classfull addressing • Consider a block of addresses that was allocated to company B • Assume that for CIDR these addresses indicate that Company B is in Canada • But Company B is actually in Europe • Company B received its block of addresses when classfull addressing was being used. • Of course Company B does not want to change it address block • Company B’s address block 196.74.4.0/22 falls within the address block 196.74.0.0/17 • EngCo’s allocation was actually 196.74.0.0/17 except for 196.74.4.0/22

  47. Return: Aggregation of networks • 64*256=16384=232-14 196.74.0.0/18 Company B’s block of addresses 196.74.32.0/22 196.74.16.0/21 196.74. 48.0/20 1024 1024 0 48 52 4 40 28 60 12 20 64 16 56 8 44 24 36 32 196.74.0.0/18 1024 1024 0 48 52 4 40 28 60 12 20 64 16 56 8 44 24 36 32

  48. CIDR: Routing and aggregation • The three networks at EngCo might be aggregated into one forwarding table entry 196.74.0.0/18 in routers (like router 1) outside of EngCo • But company B’s allocation is inside this aggregated block • How can we use the aggregated range if it contains other networks?

  49. CIDR: Routing and aggregation • What entries do we need in the forwarding table so that • company B gets its segments • EngCo gets only the segments addressed to it • Need two entries • One entry for Company B, one entry for EngCo • Company B’s entry has a “longer” match • A packet to company B matches both entries, but will be forwarded using the entry with the “longer” match (company B) • A packet to company A will match only EngCo’s aggregated entry

  50. Allocating assigned block • When a user or organization is assigned a block of IP addresses how are those addresses assigned to the hosts and networks that are part of that organization. • Can be assigned manually and permanently using static routing • Can be assigned dynamically, address given to a particular host for a particular length of time using DHCP (Dynamic Host Configuration Protocol)

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