--- title: Response: True Unnumbered Interfaces « ipSpace.net blog description: A deep technical analysis of why 'true unnumbered interfaces' don't really exist in the IP world. The post breaks down the four reasons a router needs an... canonical: https://daily.dev/posts/response-true-unnumbered-interfaces-ipspace-net-blog-o3cxylyld twitter:card: summary_large_image twitter:site: @dailydotdev og:type: website og:site_name: daily.dev og:title: Response: True Unnumbered Interfaces « ipSpace.net blog | daily.dev og:description: A deep technical analysis of why 'true unnumbered interfaces' don't really exist in the IP world. The post breaks down the four reasons a router needs an... og:url: https://daily.dev/posts/response-true-unnumbered-interfaces-ipspace-net-blog-o3cxylyld og:image: https://api.daily.dev/og/posts/O3cxYlYLD.png og:image:alt: Response: True Unnumbered Interfaces « ipSpace.net blog og:image:width: 1200 og:image:height: 630 og:locale: en --- > ## Documentation Index > Fetch the complete documentation index at: https://daily.dev/llms.txt > Use this file to discover all available pages before exploring further. # Response: True Unnumbered Interfaces « ipSpace.net blog **[ipSpace.net Blog](https://daily.dev/sources/ipspace)** · 4 min read · 0 upvotes · 0 comments ## Summary A deep technical analysis of why 'true unnumbered interfaces' don't really exist in the IP world. The post breaks down the four reasons a router needs an address associated with an interface (source address for control-plane messages, next-hop advertisement, MAC address resolution, and packet acceptance decisions). It explains how IS-IS handles IPv6 via link-local addresses but still requires IPv4 addresses for IPv4 routing, why FRRouting 10.3 must copy the loopback IPv4 address to interfaces for IS-IS IPv4 routing, and how EBGP over unnumbered interfaces uses IPv6 LLAs as a workaround. The conclusion is that no implementation achieves truly unnumbered interfaces in IPv4 — they all borrow addresses from elsewhere — and claiming OpenFabric supports 'true unnumbered interfaces' for IPv6 is misleading since every IPv6-enabled interface already gets a link-local address. ## Full article daily.dev links to this article rather than hosting it. Read it at the original source: ## Questions this post answers ### Why does FRRouting IS-IS require copying the loopback IPv4 address to other interfaces? FRRouting 10.3 with IS-IS requires the loopback IPv4 address to be copied to routing interfaces because IS-IS for IPv4 must resolve next-hop and packet-acceptance issues. Routers need an IPv4 address on each interface to advertise reachability, provide next-hop information to adjacent routers, and decide whether to accept incoming IPv4 packets — none of which can be bypassed the way IS-IS bypasses Layer 3 for its own control plane. _Network engineers running FRRouting IS-IS deployments track implementation quirks like this on daily.dev._ ### How does EBGP over unnumbered interfaces handle IPv4 next hops without an IPv4 address on the interface? EBGP over unnumbered interfaces establishes sessions between IPv6 link-local addresses, advertises IPv4 prefixes with IPv6 next hops, and uses implementation-dependent mechanisms to translate the IPv6 LLA next hop into a MAC address for forwarding IPv4 traffic. The interface still needs an IPv4 address borrowed from another interface (e.g., a loopback) to handle ICMP messages and packet acceptance decisions. _Routing engineers choosing between unnumbered interface strategies find the trade-offs covered on daily.dev._ ## Similar posts on daily.dev - [The Never-Ending IPv6 Loopback Prefix Saga « ipSpace.net blog](https://daily.dev/posts/the-never-ending-ipv6-loopback-prefix-saga-ipspace-net-blog-4nlhgh10l) · ipSpace.net Blog · 0 upvotes · 0 comments - [The world in which IPv6 was a good design](https://daily.dev/posts/the-world-in-which-ipv6-was-a-good-design-tj3i8ov0s) · Hacker News · 0 upvotes · 0 comments - [UDP Broadcasting And The Brave New World Of IPv6](https://daily.dev/posts/udp-broadcasting-and-the-brave-new-world-of-ipv6-kbr2qsudb) · Hackaday · 0 upvotes · 0 comments --- Tags: [#networking](https://daily.dev/tags/networking) [View this post on daily.dev](https://daily.dev/posts/response-true-unnumbered-interfaces-ipspace-net-blog-o3cxylyld) ```json {"@context":"https://schema.org","@graph":[{"@type":"Organization","@id":"https://daily.dev/#organization","name":"daily.dev","url":"https://daily.dev","logo":{"@type":"ImageObject","url":"https://daily.dev/apple-touch-icon.png","width":180,"height":180},"sameAs":["https://twitter.com/dailydotdev","https://github.com/dailydotdev","https://www.linkedin.com/company/daily-dev-ltd"]},{"@type":"WebSite","@id":"https://daily.dev/#website","url":"https://daily.dev","name":"daily.dev","publisher":{"@id":"https://daily.dev/#organization"},"potentialAction":{"@type":"SearchAction","target":{"@type":"EntryPoint","urlTemplate":"https://daily.dev/search?q={search_term_string}"},"query-input":"required name=search_term_string"}}]} {"@context":"https://schema.org","@type":"TechArticle","headline":"Response: True Unnumbered Interfaces « ipSpace.net blog","url":"https://daily.dev/posts/response-true-unnumbered-interfaces-ipspace-net-blog-o3cxylyld","mainEntityOfPage":{"@type":"WebPage","@id":"https://daily.dev/posts/response-true-unnumbered-interfaces-ipspace-net-blog-o3cxylyld"},"datePublished":"2026-08-11T08:52:12.210Z","dateModified":"2026-08-11T09:01:28.867Z","description":"A deep technical analysis of why 'true unnumbered interfaces' don't really exist in the IP world. 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Routers need an IPv4 address on each interface to advertise reachability, provide next-hop information to adjacent routers, and decide whether to accept incoming IPv4 packets — none of which can be bypassed the way IS-IS bypasses Layer 3 for its own control plane. Network engineers running FRRouting IS-IS deployments track implementation quirks like this on daily.dev."}},{"@type":"Question","name":"How does EBGP over unnumbered interfaces handle IPv4 next hops without an IPv4 address on the interface?","acceptedAnswer":{"@type":"Answer","text":"EBGP over unnumbered interfaces establishes sessions between IPv6 link-local addresses, advertises IPv4 prefixes with IPv6 next hops, and uses implementation-dependent mechanisms to translate the IPv6 LLA next hop into a MAC address for forwarding IPv4 traffic. The interface still needs an IPv4 address borrowed from another interface (e.g., a loopback) to handle ICMP messages and packet acceptance decisions. Routing engineers choosing between unnumbered interface strategies find the trade-offs covered on daily.dev."}}]} ```