CCNP Route - BGP part 1: Internet Connectivity

 - CCNP Route - BGP part 1: Internet Connectivity
 - CCNP Route - BGP part 2: BGP Intro
 - CCNP Route - BGP part 3: eBGP
 - CCNP Route - BGP part 4: iBGP
 - CCNP Route - BGP part 5: BGP path control
 - CCNP Route - BGP part 6: Case Studies, Labs, FAQs

The original design for the Internet - is the assignment of globally unique IPv4 addresses for all hosts connected to the Internet (not overlap with other hosts).

The assignment of a single classful network to each organization actually helped keep Internet routers’ routing tables small.
Over time, the Internet grew tremendously. It became clear by the early 1990s that something had to be done, or the growth of the Internet would grind to a halt:
 - all public IP networks would soon be assigned, and growth would be stifled,
 - routing tables in Internet routers were becoming too large for the router technology of that day.

The short-term solutions to these problems included:
CIDR - Reduce the number of wasted public IP addresses by using classless IP addressing when assigning prefixes—assigning prefixes/lengths instead of being restricted to assigning only Class A, B, and C network numbers.
NAT - Reduce the need for public IP addresses by using Port Address Translation (PAT, also called NAT overload) to multiplex more than 65,000 concurrent flows using a single public IPv4 address.
Summarization - Reduce the size of IP routing tables by making good choices for how address blocks are allocated to ISPs and end users, allowing for route summarization on a global scale.

Public IP Address Assignment
The Internet Corporation for Assigned Network Numbers (ICANN, owns the processes by which public IPv4 (and IPv6) addresses are allocated and assigned. A related organization, the Internet Assigned Numbers Authority (IANA, carries out many of ICANN policies.
These organizations define which IPv4 addresses can be allocated to different geographic regions, in addition to managing the development of the Domain Name System (DNS) naming structure and new Top Level Domains (TLD), such as .com.

Public IPv4 address assignment:
1) ICANN and IANA group public IPv4 addresses by major geographic region.
2) IANA allocates those address ranges to Regional Internet Registries (RIR).
3) Each RIR further subdivides the address space by allocating public address ranges to National Internet Registries (NIR) or Local Internet Registries (LIR are typically ISPs.)
4) Each type of Internet Registry (IR) can assign a further subdivided range of addresses to the end user organization to use.
No more unused /8 in IPv4 address space left
ICANN and IANA maintain a set of currently unallocated public IPv4 addresses. (See, and look for the IPv4 addresses link, to see the current list.)
RIRs (
Internet Route Aggregation
Although the capability to assign small blocks of addresses helped extend the IPv4 public address space, this practice also introduced many more public subnets into the Internet, driving up the number of routes in Internet routing tables.
The solution was, and still is today, to allocate numerically consecutive addresses–addresses that can be combined into a single route prefix/length—by geography and by ISP.
These allocations significantly aid route summarization.
If you are in the market for BGP routing systems, you should probably factor in a likely eBGP FIB table size of one million entries across a five year operational timespan of the equipment. (
#sh bgp summ
Wed Mar 12 22:47:14.239 EET
Neighbor  St/PfxRcd
xxx       481207
yyy       478501
zzz       481366
NAT probably has a bigger positive impact because it enables an Enterprise to use such a small number of public addresses. NAT allows an Enterprise to use private IP addresses for each host, plus a small number of public addresses.
NAT function translates the IP address from the private address (called an inside localaddress by NAT) into a public address (called an inside global address).
NAT, PAT, and NAT overload are used synonymously.
NAT-PAT-NAT Overload

Private IPv4 Addresses and Other Special Addresses
When allocating the public IPv4 address space, IANA/ICANN restricts themselves in several ways:
 - the private IP address ranges cannot be assigned to any group for use in the public Internet,
 - several other number ranges inside the IPv4 address space, as summarized in RFC 3330, are reserved for various reasons.

Private IP Address Reference (RFC1918) -     16,777,216     single class A network ( -     1,048,576     16 contiguous class B networks ( -     65,536     256 contiguous class C networks (
Dedicated space for Carrier Grade NAT deployments (RFC 6598) (4'194'302 IPs) for use in carrier grade NAT scenarios 
Reserved Values in Special-Use IPv4 Addresses (RFC 3330) - Used for self-identification on a local subnet. - Loopback testing - This “link local” block is used for default IPv4 address assignment when DHCP process fails. - Reserved for use in documentation and example code. - Used for IPv6 to IPv4 relay (6to4 relay) (RFC 3068). - Benchmark testing for Internet devices (RFC 2544) - This block, corresponding to the numericallyhighest of the former Class C addresses, was initially and is stillreserved by the IANA