Question 1

How many addresses does an IPv6 /64 subnet have?

Accepted Answer

An IPv6 /64 subnet contains 2^64 = 18,446,744,073,709,551,616 addresses (approximately 18.4 quintillion). The /64 is the standard subnet size for a single network segment because it is required for SLAAC (Stateless Address Autoconfiguration) and EUI-64 interface identifier generation.

Question 2

What is the difference between a /48, /56, and /64 in IPv6?

Accepted Answer

ISPs typically assign a /48 to end customers (65,536 subnets of /64 each). A /56 gives 256 subnets of /64. A /64 is a single network segment. Home users often receive a /56 or /64 from their ISP; enterprises receive a /48. The first 48 bits are the global routing prefix, bits 49-64 are the subnet ID, and bits 65-128 are the interface identifier.

Question 3

What is a ULA address in IPv6?

Accepted Answer

ULA (Unique Local Address) is the IPv6 equivalent of RFC 1918 private address space. ULA addresses start with fc00::/7 (in practice fd00::/8 is used). They are not routable on the public internet and are used for internal networks. The fd00::/8 range requires a randomly generated 40-bit Global ID to ensure uniqueness.

Prefix	Total Addresses	Typical Use
/32	2⁹⁶ ≈ 7.9×10²⁸	ISP allocation from RIR; top-level routing prefix
/40	2⁸⁸ ≈ 3.1×10²⁶	Large enterprise or regional ISP allocation
/48	2⁸⁰ ≈ 1.2×10²⁴	End-site allocation; gives 65,536 × /64 subnets
/52	2⁷⁶ ≈ 7.5×10²²	Mid-size enterprise sub-allocation
/56	2⁷² ≈ 4.7×10²¹	Home/small office ISP delegation; 256 × /64
/60	2⁶⁸ ≈ 2.9×10²⁰	Small site; 16 × /64
/64	2⁶⁴ = 18,446,744,073,709,551,616	Single LAN segment — standard size; required for SLAAC/EUI-64
/96	2³² = 4,294,967,296	IPv4-mapped/compatible addresses (::ffff:0:0/96)
/112	2¹⁶ = 65,536	Point-to-multipoint; sometimes used for WAN links
/126	4	Point-to-point links (RFC 3627 — analogous to IPv4 /30)
/127	2	Point-to-point links (RFC 6164 — analogous to IPv4 /31, preferred)
/128	1	Single host route, loopback (::1/128)

Prefix	Type	IPv4 Equivalent	Notes
::1/128	Loopback	127.0.0.1	Every host has this address; never routed
fe80::/10	Link-local	169.254.0.0/16	Auto-configured; valid only on the local link; required on every interface
fc00::/7	Unique Local (ULA)	10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16	Not internet-routable; fd00::/8 is the usable range with random Global ID
2000::/3	Global Unicast (GUA)	Public IP addresses	Internet-routable; all current public IPv6 space is in this range
ff00::/8	Multicast	224.0.0.0/4	One-to-many delivery; ff02::1 = all nodes on link
2001:db8::/32	Documentation	192.0.2.0/24 etc.	Reserved for examples and documentation (RFC 3849); not routable
::ffff:0:0/96	IPv4-mapped	—	Represents IPv4 addresses in IPv6 sockets
64:ff9b::/96	IPv4-translated	—	Used by NAT64 gateways for IPv4/IPv6 translation

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