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Internet Networking recitation #1

Internet Networking recitation #1. Subnet + CIDR. Administrative Information. Course site: webcourse.cs.technion.ac.il/236341 Grading policy: 15% homeworks + 85% final exam 6 home assignments.  Submission is in pairs. Assistants: Anna Levin Phone:   (829)4306

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Internet Networking recitation #1

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  1. Internet Networkingrecitation #1 Subnet + CIDR Spring Semester 2009, Dept. of Computer Science, Technion

  2. Administrative Information • Course site: webcourse.cs.technion.ac.il/236341 • Grading policy: • 15% homeworks + 85% final exam • 6 home assignments.  • Submission is in pairs. • Assistants: • Anna Levin • Phone:   (829)4306 • Office location:   Taub 323 • Roman Sandler • Phone:   (829)4166 • Office location:   Taub 736 Internet Networking

  3. IP Addressing: Original Classful Scheme • IP Address – 32-bit integer globally unique address • Dotted Notation: 132.68.37.54 • IP Classes – dividing an address to net id and host id • The prefix (net id) identifies a network. • The suffix (host id) identifies a host on this network. Internet Networking

  4. IP Addressing: Original Classful Scheme • Class A – 7 bits to net id, 24 bits to host id 1.0.0.0 – 126.0.0.0 • Class B – 14 bits to net id, 16 bits to host id 128.0.0.0 – 191.255.0.0 • Class C – 21 bits to net id, 8 bits to host id 192.0.0.0 – 223.255.255.0 • Class D – for multicasting • Class E – reserved for future use (used for private addresses) Weakness • Growth of routing tables in routers • Tens of thousands small (class C) networks. • Each network must be advertised. • Inflexible • Lack of a network classes for mid-sized organization (between class B and C). • Address space will be eventually exhausted Internet Networking

  5. Subnet Addressing • A site has a single IP network address assigned to it, but has two or more physical networks. • Different technologies. • Limits of technologies. • Network congestion. • Security consideration. • VLAN – separate one physical network into a few logical networks. • Administration (e.g. different departments in academic institute). • From outside it looks like a single network • Only local routers know about multiple physical networks inside and how to route traffic among them • Host ID is divided into a subnet ID and host ID • Accepted as a standard at 1985 (RFC 950). Internet Networking

  6. Subnet Routing • When a router gets a packet, it isolates by Net mask the packet net id address. • Each routing entry contains a net mask. • Routing is done on a longest-match basis. • If the packet is destined to other network then the router sends it to another router. • Otherwise the router sends the packet to the appropriate host on its attached networks. Internet Networking

  7. Subnetting - Example • A site with two physical networks. • Using subnetting, R advertise these networks as a single network (thus, R accepts all traffic for net 128.10.0.0) • Internal routing is done according to subnet id (i.e. the third octet of the address). Network 128.10.1.0/24 128.10.1.1 128.10.1.2 H1 H2 Rest of the Internet R . Network 128.10.2.0/24 All traffic to 128.10.0.0/16 128.10.2.1 128.10.2.2 H3 H4 Internet Networking

  8. Variable-Length Subnetting • Motivation: Consider the case when an organization has a few networks of different sizes. • When we choose the subnet partitioning, we actually define constant number of possible physical subnetworks with maximum number of hosts on them. • Difficult to keep small (waist of subnet numbers) and big (the host id needs more bits) sub networks and there could be unnecessary spending of address space. • Solution: Variable-Length Subnetting. A subnet partition is selected on a per-network basis. Internet Networking

  9. Example – Configuring a Network withVariable-Length Subnetting • We have a network with IP 202.128.236.0/24 • We need to support next sub networks: • 6 networks with 26 hosts • 3 networks with 10 hosts • 4 networks with 2 hosts • If we take subnet mask of /27 bits then we can get 8 sub networks of 30 hosts (all 0’s and all 1’s of host addresses are reserved). • 11111111.11111111.11111111.11100000 • We need only 6 such sub networks. • The rest 2 sub networks we will partition by subnet mask of /28 bits. • 11111111.11111111.11111111.11110000 • We will get 4 sub networks of 14 hosts in each • We need only 3 such sub networks. Internet Networking

  10. Example – Configuring a Network withVariable-Length Subnetting • The rest we will partition by subnet mask of /30 bits. • 11111111.11111111.11111111.11111100 • We will get 4 sub networks of 2 hosts in each. • Subnet mask #1 = /27 • 11111111.11111111.11111111.11100000 • Subnet mask #2 = /28 • 11111111.11111111.11111111.11110000 • Subnet mask #3 = /30 • 11111111.11111111.11111111.11111100 Internet Networking

  11. Classless Inter-Domain Routing (CIDR) - RFC 1519 • Routing destinations are represented by network and mask pairs. • Enabling network aggregation; thereby reducing the size of routing table. Examples: • Class A networks are followed by a /8 • Class C networks are followed by a /24 • 8 Class C hosts network is followed by /21 • Such a network has 21 bits of Net-ID, 11 Bits of Host-ID • Contains 2^21 Net IDs, and 2^11-2 = Hosts in Each network. Internet Networking

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