diff --git a/cmd/mesh-bootstrap/main.go b/cmd/mesh-bootstrap/main.go
index adb27ba..f8ae993 100644
--- a/cmd/mesh-bootstrap/main.go
+++ b/cmd/mesh-bootstrap/main.go
@@ -18,6 +18,7 @@ package main
 import (
 	"context"
 	"encoding/json"
+	"errors"
 	"flag"
 	"fmt"
 	"io"
@@ -239,6 +240,18 @@ func registryAgrees(set *flag.FlagSet, opts *bootstrap.Options) error {
 	said := map[string]bool{}
 	set.Visit(func(f *flag.Flag) { said[f.Name] = true })
 	host, portText, err := net.SplitHostPort(opts.Registry)
+	var missing *net.AddrError
+	if errors.As(err, &missing) && missing.Err == "missing port in address" {
+		// A host alone, as --registry took before its port became the node's: the registry's
+		// port — the one given, or the catalogue's — completes it.
+		port := opts.Ports.Registry
+		if port == 0 {
+			port = bootstrap.DefaultPorts().Registry
+		}
+		opts.Ports.Registry = port
+		opts.Registry = net.JoinHostPort(strings.Trim(opts.Registry, "[]"), strconv.Itoa(port))
+		return nil
+	}
 	if err != nil {
 		return fmt.Errorf("--registry %q is not host:port: %w", opts.Registry, err)
 	}
diff --git a/cmd/mesh-bootstrap/main_test.go b/cmd/mesh-bootstrap/main_test.go
index 2a4a4bb..8d19d6c 100644
--- a/cmd/mesh-bootstrap/main_test.go
+++ b/cmd/mesh-bootstrap/main_test.go
@@ -190,3 +190,17 @@ func TestTheRegistrysPortAndAddressAgree(t *testing.T) {
 		t.Error("two ports for one registry were accepted")
 	}
 }
+
+func TestARegistryGivenAsAHostAloneTakesTheRegistrysPort(t *testing.T) {
+	_, opts, _, err := parseArgs([]string{"--registry", "192.0.2.10"})
+	if err != nil || opts.Registry != "192.0.2.10:5000" || opts.Ports.Registry != 5000 {
+		t.Errorf("--registry host alone: %s %d %v", opts.Registry, opts.Ports.Registry, err)
+	}
+	_, opts, _, err = parseArgs([]string{"--registry", "192.0.2.10", "--registry-port", "5100"})
+	if err != nil || opts.Registry != "192.0.2.10:5100" {
+		t.Errorf("--registry host with --registry-port: %s %v", opts.Registry, err)
+	}
+	if _, _, _, err := parseArgs([]string{"--registry", "192.0.2.10:notaport"}); err == nil {
+		t.Error("a registry with a port that is not a number was accepted")
+	}
+}