• A site's IP address block is allocated by its ISP
• Network admins can use all the address space
• An address plan breaks the allocation into subnets

• A router allows hosts to communicate to other IP devices not on the local subnet
• PointConnection between subnet and external network
• Router interface requires its own IP address (Within the subnet)
• Typically the first or last available

• Defines how your address spac eis used
• IPv4 space efficiency
• Maintain a high cost density ratio

Host subnets are all /64

• e.g. if 2001:630:d0:80::/60 were allocated, we could have:
• Labs 1: 2001:630:d0:80::/64
• Labs2: 2001:630:d0:81::/64
• Offices1: 2001:630:d0:82::/64
• Offices2: 2001:630:d0:83::/64

• A package can be sent directly to recipient if it is on the same local subnet
• If destination is not on the local subnet, forward the packet to a router fo the subnet that has external connectivity.

• Handles all communications to/from the IP subnet
• Usually called the default router
• IPv4 hosts usually learn the router's address by DHCP
• DHCP server offers host an IP address and a lease timer
• IPv6 hosts can use stateless autoconfiguration
• Router's address is learnt from Router Advertisements

• IP address
• Network mask (netmask)
• Default router
• DNS resolver

• Used to determine if destination is local
• Perform a bitwise AND of IP and netmask
• If resulting network/subnet prefix is the same for both, then the destination is the same subnet
• Netmask is configured by DHCP in IPv4
• Netmask is effectively always 64-bit in IPv6

• If too 'large' hosts will try to communicate directly when they cannot
• If too 'small' hosts will send to the router when they should send locally

eg. (Large) If netmask should be 255.255.255.0 and is set to 255.255.255.128, hosts will think the destination is in a different subnet when it's actually in the same one

eg. (Small) If netmask should be 255.255.255.0 and is set to 255.255.254.0, hosts will think some destinations are local, when in fact they are beyond the router

What does this Linux IPv4 routing table represent?

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In this example the network prefix is a /26 (first 26 bits of netmask are set). Network address is 255.255.255.192 AND 152.78.68.162, ie. 152.78.68.128. Thus the network prefix is 152.78.68.128/26

The routing table says

"Put anything for 152.78.68.128/26 (the local subnet) out on eth0 (my Ethernet interface)"

"Route everything else (default route is 0.0.0.0) via the default router with IP address 152.78.68.190, out on eth0 (my Ethernet interface)"

• Packets are sent externally via the default router
• Default router needs to know the next router or subnet/link to forward the packet to
• May be many hops (routers) from source to dest
• Routing protocols come into play
• Routers exchange reachability information for specific networks (prefixes) using routing protocols
• Different protocols need intra-site or inter-site

• Static routes
• Configure all routes manually
• Problematic if topology changes or network faults occur
• Dynamic routing (More common + practical)
• Several protocols

• IS-IS - link state algorithm
• Open Shortest Path First (OSPF)- Link state algorithm
• Routing Information Protocol (RIP) - distance vector algorithm